System and method for hole inspection and qualification

ABSTRACT

A system and method are provided for performing a hole inspection performance study. Specimens for the performance study are made from a reconfigurable set of inspection plates. Each plate includes multiple test holes which are located symmetrically. The plates may be of various thicknesses and materials. Each test hole may or may not have a feature such as a crack or machining notch. Such features may be located at various positions of the hole, such as at an edge, within the bore, and at various circumferential positions. A specimen is formed by stacking two or more plates and securing the stack together with an alignment tool. A variety of specimens may be formed by using different combinations of inspection plates and flipping and rotating the member plates. A hole inspection system is disclosed as well as an inspection procedure and data processing algorithm for inspecting each hole.

RELATED APPLICATION

The present application claims priority under 35 U.S.C. § 119(e) to U.S.provisional patent application, U.S. Ser. No. 63/180,317, filed Apr. 27,2021 which is herein incorporated by reference in its entirety.

GOVERNMENT SUPPORT

The invention was supported, in whole or in part, under contractFA8571-19-C-A015, from the U.S. Air Force. The Government has certainrights in the invention.

TECHNICAL FIELD

The present disclosure relates to the field of non-destructiveevaluation (NDE).

BACKGROUND

NDE can be used to evaluate the condition of materials at and duringproduction, prior to deployment, periodically during deployment, andafter deployment. In some applications, NDE of holes, such as boltholes, is critical to determining whether a part can continue to besafely used, or if it should be repaired or replaced (or simply usedless aggressively).

In U.S. Pat. No. 10,677,756, issued on Jun. 9, 2020, which isincorporated herein by reference in its entirety, Goldfine et al.(hereinafter “Goldfine I”) described “an integrated mandrel” forinspecting bolt holes. The mandrel included a sensor such as an eddycurrent array, and mechanical support to facilitate hole inspection. Ahandheld scanner was contemplated for use with the mandrel and a sensorwas described. Goldfine I's mandrel may include a mechanical wedge or aballoon portion that can be inflated (using gas or liquid) to applypressure against the sensor to the bolt hole. A piston was described foractuation. Goldfine I described various scanner mechanisms to facilitaterotation of the mandrel within the bolt hole such as a slip ring scannerand the use of a spooling connection tape. A portion of the sensormeasurement electronics is located on the mandrel (rotating) side whilethe remaining portion of the sensor measurement electronics are locatedon the stationary side.

In US. Patent Pub. No. 2021/0055262, published Feb. 25, 2021, which isincorporated herein by reference in its entirety, Goldfine et al.(hereinafter “Goldfine II”) describes a process for enhancing detectionof defects having characteristic shapes provided in a signature library.Signatures, which may be obtained from actual sensor measurements from aknown defect and then correlated with sensor measurements. A largecorrelation may be an indication that a defect is present at thematerial location where the measurement data was collected. Goldfine IIfurther describes methods for obtaining signatures for the signaturelibrary, selecting an appropriate signature from the library for data,performing single and multichannel correlation, and flagging defectdetections.

SUMMARY

A system and method are provided for performing a hole inspectionperformance study. Specimens for the performance study are made from areconfigurable set of inspection plates. Each plate includes multipletest holes which are located symmetrically. The plates may be of variousthicknesses and materials. Each test hole may or may not have a featuresuch as a crack or machining notch. Such features may be located atvarious positions of the hole, such as at an edge, within the bore, andat various circumferential positions. A specimen is formed by stackingtwo or more plates and securing the stack together with an alignmenttool. A variety of specimens may be formed by using differentcombinations of inspection plates and flipping and rotating the memberplates. A hole inspection system is disclosed as well as an inspectionprocedure and data processing algorithm for inspecting each hole.

One aspect relates to an apparatus comprising a test specimen having aplurality of inspection plates, each plate having a plurality of testholes symmetric about a first axis of said plate; and an alignment toolsecuring the plurality of inspection plates such that corresponding testholes on each plate are axially aligned, wherein a first plate among theplurality of inspection plates has a defect feature at a first test holeamong the plurality of test holes for said first plate.

In some embodiments, the plurality of test holes on each of theplurality of inspection plates comprises a first plurality of test holesat a first radius from the first axis and a second plurality of testholes at a second radius from the first axis, the second radius greaterthan the first radius. In some embodiments, the first plurality of testholes have a first hole diameter and the second plurality of test holeshave a second hole diameter different from the first hole diameter.

In some embodiments, the plurality of test holes on each of theplurality of inspection plates are each at a first radius from the firstaxis and are equal angularly spaced about the first axis.

In some embodiments, each of the plurality of inspection plates has afirst face, a second face and an exterior edge, the exterior edge havingan indicator identifying which face is the first face.

In some embodiments, the plurality of test holes on each plate of theplurality of inspection plates are symmetric about a second axis of saidplate, the first and second axes being perpendicular to one another.

In some embodiments, the plurality of plates have a same thickness.

In some embodiments, the plurality of inspection plates of the testspecimen are a sub-plurality of a test set of inspection plates and allplates in the test set have test holes.

In some embodiments, the defect feature is of a type selected from agroup consisting of a crack and an electro discharge machining (EDM)notch. In some embodiments, the defect feature is at a corner locationof the first test hole. In some embodiments, the defect feature ismidbore in the first test hole.

In some embodiments, each of the plurality of inspection plates hasunique identifying markings along its edge, and the test specimenfurther comprises an edge blocker that surrounds the perimeter of theplurality of inspection plates such that the plates' edges are notvisible.

Another aspect relates to an apparatus comprising a test specimen havinga plurality of inspection plates, each plate having a plurality of testholes symmetric about a first axis of said plate and symmetric about asecond axis of said plate, the first and second axes being perpendicularto one another; and an alignment tool securing the plurality ofinspection plates such that corresponding test holes on each plate areaxially aligned, wherein a first plate among the plurality of inspectionplates has a defect feature at a first test hole among the plurality oftest holes for said first plate.

In some embodiments, the plurality of inspection plates further includea plurality of alignment holes symmetric about the first axis of saidplate and symmetric about the second axis of said plate; and thealignment tool includes a plurality of fasteners, each fastener passingthrough a respective alignment hole of each of the plurality ofinspection plates. In some embodiments, the alignment tool furtherincludes a base to which the plurality of inspection plates are securedby the plurality of fasteners. In some embodiments, each of theplurality of inspection plates is approximately rectangular in shape,has on a first edge a first marking offset from the first axis by afirst amount, and has on a second edge, opposite the first edge, asecond marking offset from the first axis by a second amount less thanthe first; and the base has four orientation indicators positioned onthe base such that for each plate either the respective first marking orthe respective second marking aligns with one of the four orientationindicators depending on the orientation of said plate.

In some embodiments, the alignment tool includes a plurality offasteners, each fastener passing through a respective test hole of eachof the plurality of inspection plates, the plurality of fasteners beingof a smaller number than a number of axially aligned test holes in thespecimen.

Another aspect relates to a method of determining performance of a holeinspection system, the method comprising acts of (i) providing a testset of inspection plates, each plate in the test set having a pluralityof test holes symmetric about a first axis of said plate, a first platein the test set having a defect feature at a first test hole among itsplurality of test holes; (ii) securing with an alignment tool a firstsubset of plates in the test set into a first test specimen such thatcorresponding test holes on each plate in the first subset are axiallyaligned, the first subset of plates including the first plate; (iii)performing an inspection procedure with the hole inspection system ineach of the plurality of test holes in the first test specimen; (iv)storing inspection results for each of the plurality of test holes inthe first test specimen; and (v) determining performance of the holeinspection system based at least in part on whether the results detectedthe defect feature in the first test hole of the first plate.

In some embodiments, the method further comprises acts of (vi) securingwith the alignment tool a second subset of plates in the test set into asecond test specimen, wherein the second subset of plates is differentfrom the first subset in at least one of (a) an order of plates, (b) aplate common to both the first and second subsets of plates is rotatedabout its first axis in the second test specimen relative to the firsttest specimen, (c) a plate common to both the first and second subsetsof plates is flipped over in the second test specimen relative to thefirst test specimen, (d) a plate in the first subset is absent from thesecond subset, and (e) a plate in the second subset is absent from thefirst subset; and (vii) repeating acts (iii) and (iv) for the secondtest specimen, wherein act (v) is performed based on inspection resultsfrom both the first and second test specimens.

In some embodiments, the method further comprises acts of (vi) securingwith the alignment tool a second subset of plates in the test set into asecond test specimen, wherein the second subset of plates is differentfrom the first subset in at least a plate common to both the first andsecond subsets of plates is flipped over in the second test specimenrelative to the first test specimen; and (vii) repeating acts (iii) and(iv) for the second test specimen, wherein act (v) is performed based oninspection results from both the first and second test specimens.

Yet another aspect relates to a method for detecting a feature on theedge of a test material layer where said layer has an upper and loweredge and the two edges are a essentially parallel, the method comprisingacts of (i) scanning an eddy current sensor in a direction essentiallyparallel to at least one edge, where the eddy current sensor iscomprised of a linear drive conductor and at least two sensing elementsat the same distance from the linear drive; (ii) recording the responseof the two sensing element simultaneously; (iii) scanning the sensoralong a length parallel to the at least one edge covering a distancerequiring inspection, then incrementing the sensor in the directionperpendicular to the at least one edge, where the increment is equal tothe scan width covered by the at least two sensing elements minus anoverlap region that is between 50% and 250% of the width of one sensingelement in the direction of incrementing; (iv) estimating the locationof the upper and the lower edge from the response of at least onesensing element; (v) storing on a non-transient computer-readablestorage medium a set of inspection datapoints for a plurality scandirection locations along the edge; (vi) operating a processor tocompute a metric value that represents a quality of a match between atleast a subset of the inspection datapoints and a reference signaturefor the feature, the reference signature having a plurality of referencesignature datapoints; and (vii) comparing the metric value to athreshold value to determine whether the feature is present within thedistance requiring inspection for the test material having a secondlinear conductor at the same distance from the sensing elements on theopposite side of the sensing elements, orienting the drive conductor atapproximately 45 degrees relative to at least one edge, and maintainingsymmetry in the sensor design so that when the sensor is scanned alongthe upper edge the response to the feature at the upper edge from atleast one sensing element matches the response from at least one sensingelement to the feature at the lower edge, enabling the use of a singlesignature in the signature library for the same feature when located ateither the upper or lower edge.

In some embodiments, each inspection datapoint in the scan direction isassociated with a spatial location within a hole in a multiple layerstack-up and the two edges are the upper and lower edges of a layer inthe multiple layer stack-up.

In some embodiments, the stack-up is comprised of at least two layersand the method further comprises estimating the electrical conductivityof each layer using a multivariate inverse method that uses aprecomputed database of sensor responses to estimate both theconductivity and liftoff to ensure that the conductivity value isindependent of the liftoff; where the conductivity and liftoff valuesare used to select the signature from a signature library.

In some embodiments, the feature is a crack. In some embodiments, thefeature is a pit. In some embodiments, the feature is a scratch.

Yet another aspect relates to a method for detecting a feature in a testmaterial where a signature library is generated from a set of at leasttwo responses derived from empirical scans of a second test materialwith at least one similar known feature, the method comprising acts of(i) scanning a sensor with a drive and at least two sensing elements,where the drive produces in interrogating field and the sensing elementsrespond to at least one material property of the test material, wherethe response of the two sensing elements are recorded simultaneously;(ii) scanning the sensor along a length covering a distance requiringinspection, then incrementing the sensor in the direction perpendicularto the scan direction, where the increment is equal to the scan widthcovered by the at least two sensing elements minus an overlap regionthat is between 50% and 250% of the width of one sensing element in thedirection of incrementing; (iii) storing on a non-transientcomputer-readable storage medium a set of inspection datapoints; (iv)operating a processor to compute a metric value that represents aquality of a match between at least a subset of the inspectiondatapoints and a reference signature from the signature library for thefeature, the reference signature having a plurality of referencesignature datapoints; and (v) comparing the metric value to a thresholdvalue to determine whether the feature is present within the distancerequiring inspection for the test material.

In some embodiments, each inspection datapoint in the scan direction isassociated with a spatial location within a hole in a multiple layerstack-up.

In some embodiments, the signature library is generated using a set ofsamples with simulated defects at multiple locations to produce asubstantial number of signatures that represent each of the likelylocations for the feature within the test material. In some embodiments,the simulated defect is an EDM notch and the feature of interest is acrack, and where the scans that generate the signature library includescans with the EDM notch at various locations in the transverse,channel, direction to enable rescaling of responses to account forvaried position of the notch within the sensor array width.

In some embodiments, the signature library is tested against a trainingset with at least 7 representative defects for the feature of interestat at least two different locations within a test sample that has asimilar geometry to the part geometry to be inspected, and the testingthe signature library comprises building a probability of detectioncurve for the signature library and comparing the test results for aminimum desired defect size to a performance goal.

In some embodiments, the time for processing the data is also measuredand the number of signatures in the signature library is reduced untilthe time for processing is not more than the time for inspection. Insome embodiments, the geometry to be inspected is a hole, the defect isa crack and the time to process the data is less than the time toinspect a single hole. In some embodiments, the number of signatures inthe library is increased to achieve sufficient POD performance, wherethe added signatures come from scans of the test set and the test setsamples from which the signatures where derived are then removed fromthe test set and replaced with other samples to keep the number ofsamples above 7 in the test set.

One aspect relates to a sensor cartridge having a control wheel; a shafthaving an axis that is co-axial with the control wheel; an expansionelement co-axial with the shaft, having a helical portion, a firstlocation and a distal end, wherein the expansion element is operablyfixed to the control wheel at a first location and operably fixed to adistal end of the shaft at the distal end of the expansion element; anda sensor having a sensing array portion, the sensing array portionattached to the helical portion of the expansion element.

In some embodiments the control wheel is toothed around itscircumference.

In some embodiments the sensor further comprises a lead portion and thesensor cartridge further comprises a reel co-axial to the shaft forwinding the lead portion of the sensor. The reel may be positioned alongthe axis between the control wheel and the helical portion of theexpansion element. The sensor cartridge may also include a bearinghaving an inner ring, wherein the inner ring is around the reel. Thesensor cartridge may also include a circular disc co-axial with theshaft and having a hole therein through which the shaft passes, thecircular disc operably fixed to the control wheel on one side, andoperably fixed to the reel on an opposite side. In some embodiments theexpansion element, reel, and circular disc are a single piece ofmaterial. For example, the expansion element, reel and circular disc maybe fabricated by an additive manufacturing process.

In some embodiments of the sensor cartridge the helical portioncomprises a first helix and a second helix, and the sensing arrayportion is attached to the first helix of the helical portion. A firstwidth of the first helix may be greater than a second width of thesecond helix, the first and second widths being measured in an axialdirection defined by the shaft. The first helix may have an inset intowhich a sensor array portion of the sensor is attached.

In some embodiments the sensor cartridge is part of an apparatus thatfurther comprises a scanner having a second control wheel (the sensorcartridge's control wheel being the first control wheel), a motorconnected to the second control wheel, and a bearing having inner andouter rings. The sensor cartridge may be secured within the scanner withthe first control wheel engaged with the second toothed wheel, and thesensor cartridge passes through the inner ring of the scanner's bearing.

In some embodiments, the sensor cartridge further comprises a nutoperably fixed to a proximal end of the shaft. The sensor cartridge maybe part of an apparatus that further comprises a scanner having a pawl,a second control wheel (the sensor cartridge's control wheel being thefirst control wheel), a motor connected to the second toothed wheel, anda bearing having inner and outer rings. The sensor cartridge may besecured within the scanner with the first toothed wheel engaged with thesecond toothed wheel; the nut may be a gear and the pawl may be engagedwith the gear, and the sensor cartridge passes through the inner ring ofthe scanner's bearing.

Another aspect relates to a method of placing a sensor within a hole ofa test piece. The method comprises acts of (i) providing a sensorcartridge having a shaft, and a expansion element with a helicalportion, the expansion element co-axial with the shaft, the expansionelement operably fixed to a distal end of the shaft at a distal end ofthe expansion element, the helical portion have a sensor affixedthereto, and the helical portion having in a relaxed state a firstradius; (ii) tightening the helix portion by applying a torque to aproximal end of the shaft relative to a proximal end of the expansionelement in a tightening direction thereby causing the helix portion tohave a second radius less than the first radius; (iii) while applyingthe torque, inserting the sensor cartridge into the hole such that thesensor is at least partially within the hole; and (iv) releasing thetorque.

In some embodiments, the hole has a hole radius, the second radius isless than the hole radius, and the first radius is equal to or greaterthan the hole radius.

In some embodiments, the method further comprises, (v) after releasingthe torque, measuring a response of the sensor.

In some embodiments, the helical portion of the provided sensorcartridge comprises at least one left handed helix and the tightening ofthe helical portion is achieved by applying a clockwise torque to thehelix portion.

In some embodiments, the helical portion of the provided sensorcartridge comprises at least one right handed helix and the tighteningof the helical portion is achieved by applying a counter-clockwisetorque to the helix portion.

In some embodiments the providing (i) further comprises providing thesensor cartridge in a motorized scanner operably connected to theproximal end of the shaft and a control wheel that is operably fixed atthe proximal end of the expansion element; and the tightening (ii)comprises fixing the proximal end of the shaft with a pawl of thescanner and operating a motor of the scanner to rotate the wheel.

Yet another aspect relates to a method of placing a sensor within a holeof a test piece, the method comprising acts of (i) providing a sensorcartridge having a shaft, and a expansion element with a helicalportion, the expansion element co-axial with the shaft, the expansionelement operably fixed to a distal end of the shaft at a distal end ofthe expansion element, the helical portion have a sensor affixedthereto, and the helical portion having in a relaxed state a firstradius; (ii) inserting the sensor cartridge into the hole such that thesensor is at least partially within the hole; and (iii) expanding thehelix portion by applying a torque to a proximal end of the shaftrelative to a proximal end of the expansion element in an expandingdirection thereby causing the helix portion to have a second radiusgreater than the first radius.

In some embodiments the hole has a hole radius and the first radius isless than the hole radius.

In some embodiments the method further comprises an act of (iv)measuring a response of the sensor while applying the torque to expandthe helix portion.

Yet another aspect relates to a method for enhancing a primary featurein a test object, the method comprising acts of (i) storing a signaturefor a secondary feature in a non-transient computer-readable storagemedium; (ii) placing a sensor having a drive conductor and at least onesense element proximate to the test object; (iii) scanning the sensoracross the test object; (iv) during the scanning operating an immittanceinstrument to excite the drive conductor and obtain sensor data bymeasuring the at least one sense element; and (v) operating a processorto (1) locate the secondary feature by calculating a correlation basedon the signature and the sensor data; and (2) suppress the secondaryfeature in the sensor data based on the correlation thereby enhancingthe primary feature.

In some embodiments, the test object has adjacent first and secondmaterial layers, the secondary feature is a fastener response due thepresence of a fastener through the first and second material layers. Insome embodiments, operating the processor to suppress the secondaryfeature comprises subtracting the signature, after scaling, from thesensor data.

In some embodiments, act (v) further comprises operating the processorto (3) prior to acts (v)(1) and (v)(2), estimate a material property ofthe test object, wherein the operating the processor to suppress thesecondary feature comprises modifying the estimate of the materialproperty based on the correlation. In some embodiments, act (v) furthercomprises operating the processor to (4) determine if the primaryfeature is present based on the modified material property estimate. Insome embodiments, the primary feature is one of a group consisting of acrack, a pit, and a scratch. In some embodiments, the test object hasadjacent first and second material layers and the modified materialproperty estimates a gap distance between the first and second materiallayers.

In some embodiments, the secondary feature is one of a group consistingof a fastener, a groove in the test object, an edge of the test object,and a thickness of an insulating coating portion of the test object.

In some embodiments, the test object has adjacent first and secondmaterial layers, and act (v) further comprises operating the processorto (3) prior to acts (v)(1) and (v)(2), estimate material properties ofthe test object including a thickness of the first material layer andelectrical conductivities of the first and second layers by utilizing amultivariate inverse method and a precomputed database, wherein in act(v)(1) the correlation based on the signature and at least one of theestimated material properties.

In some embodiments, the method further comprises storing a plurality ofsignatures for the secondary feature, the plurality of signaturesincluding the signature, and act (v) further comprises operating theprocessor to (3) prior to acts (v)(1) and (v)(2), compare the senseelement responses to a library of characteristic responses for anon-relevant feature to select a reference characteristic response andusing this reference characteristic response to adjust the sense elementresponse. In some embodiments, a characteristic response includes asense element response variation with scan position. In someembodiments, a characteristic response includes information from atleast two sense elements. In some embodiments, adjusting the senseelement response comprises subtracting the reference characteristicresponse from the sense element response for each excitation frequency.

Yet another aspect relates to a method for detecting a feature on afirst edge of a test material layer, the test material layer also havinga second edge, the method comprising acts of first scanning an eddycurrent sensor in a direction substantially parallel to the first edge,the eddy current sensor having a linear drive conductor and at least twosensing elements at a same distance from the linear drive conductor;measuring, during the first scanning, simultaneous responses of the atleast two sensing elements; moving the sensor perpendicular to the firstedge in an increment less than or equal to a scan width; second scanningand measuring the sensor in the incremented position; repeating, asnecessary, the moving and the second scanning at least until the secondedge is inspected; estimating a location of the first edge and thesecond edge from the responses of at the least one sensing element;operating a processor to compute a metric value that represents aquality of a match between at least a subset of the inspectiondatapoints and a reference signature for the feature, the referencesignature having a plurality of reference signature datapoints;comparing the metric value to a threshold value to determine whether thefeature is present within the distance requiring inspection for the testmaterial; and providing an automated report of the location of thefeature along the scan direction and relative to the first edge in thedirection perpendicular to the first edge, where the location of thefeature is determined from the responses of the at least two sensingelements. In some embodiments, the feature is a crack, or a pit, or ascratch.

In some embodiments, the location of the feature is used to determinewhich layer the feature is in which in turn verifies that the propersignature was used. In some embodiments, the proper signature isselected based on one of the following, layer conductivity, layerthickness, liftoff for the layer. In some embodiments, a gap betweenlayers is estimated.

In some embodiments, the increment is equal to a scan width covered bythe at least two sensing elements minus an overlap region that isbetween 50% and 250% of a width of one sensing element in a direction ofthe increment.

In some embodiments, the increment is estimated by comparing sequentialscans in the overlap region.

In some embodiments, the method further comprises storing on anon-transient computer-readable storage medium a set of inspectiondatapoints for a plurality scan direction locations along the firstedge;

In some embodiments, each inspection datapoint in the scan direction isassociated with a spatial location within a hole in a multiple layerstack-up and the two edges are the upper and lower edges of a layer inthe multiple layer stack-up.

In some embodiments, the stack-up is comprised of at least two layersand the method further comprises estimating the electrical conductivityof each layer using a multivariate inverse method that uses aprecomputed database of sensor responses to estimate both theconductivity and liftoff to ensure that the conductivity value isindependent of the liftoff; where the conductivity and liftoff C-Scanimages are displayed for the operator to verify that the requiredinspection area was scanned completely and throughout the scan theliftoff did not exceeding a prescribed value above which thesignal-to-noise for detection of the feature of interest is not lessthan 2, where this prescribed liftoff value is determined in advancefrom empirical tests on representative parts that include at least onerepresentative feature.

In some embodiments, the feature is a crack and the crack location isindicated on the conductivity C-Scan image as a red marking with theextent of the red marking approximately representing the extent of thedefect in both the scan direction and the direction perpendicular to theessentially straight edge.

Yet another aspect relates to a method for detecting a feature in a holein a test material, the method comprising acts of (i) storing on anon-transient computer-readable storage medium a set of inspectiondatapoints for a plurality circumferential locations within the hole;(ii) operating a processor to compute a metric value that represents aquality of a match between at least a subset of the inspectiondatapoints and a reference signature for the feature, the referencesignature having a plurality of reference signature datapoints; and(iii) comparing the metric value to a threshold value to determinewhether the feature is present within the hole in the test material.

In some embodiments, the test material has a plurality of materiallayers and the reference signature is part of a signature library havinga plurality of references signatures for the feature, and the methodfurther comprises (iv) determining an electrical conductivity of amaterial layer with which a subset of inspection datapoints isassociated based in part on the subset of inspection datapoints; and (v)selecting the reference signature from the signature library based atleast in part on the electrical conductivity of the material layer.

In some embodiments, each inspection datapoint is associated with aspatial location within the hole. In some embodiments, the set ofinspection datapoints comprises a first subset of inspection datapointsmeasured at a first excitation frequency and a second subset ofinspection datapoints measured at a second excitation frequency. In someembodiments, the reference signature is part of a signature libraryhaving a plurality of references signatures for the feature, and themethod further comprises selecting the reference signature from thesignature library based at least in part on the spatial location of atleast one inspection datapoint within the subset of the inspectiondatapoints. In some embodiments, the spatial location comprises an axiallocation and the selecting of the reference signature is based at leastin part on the axial location of the at least one inspection datapoint.In some embodiments, the spatial location comprises an axial locationand the selecting of the reference signature is based at least in parton the relative position of the axial location of the at least oneinspection datapoint to a second axial location in the hole. In someembodiments, the second axial location is at the edge of a layer in amultiple layer stackup.

In some embodiments, the test material has a plurality of materiallayers and the reference signature is part of a signature library havinga plurality of references signatures for the feature, and the methodfurther comprises (iv) determining an electrical conductivity of amaterial layer with which the subset of inspection datapoints isassociated; and (v) selecting the reference signature from the signaturelibrary based at least in part on the electrical conductivity of thematerial layer.

In some embodiments, the test material has a plurality of materiallayers and the reference signature is part of a signature library havinga plurality of references signatures for the feature, and the methodfurther comprises (iv) determining an layer thickness of a materiallayer with which the subset of inspection datapoints is associated; and(v) selecting the reference signature from the signature library basedat least in part on the layer thickness of the material layer.

In some embodiments, the test material has a plurality of materiallayers and the reference signature is part of a signature library havinga plurality of references signatures for the feature, and the methodfurther comprises (iv) determining a liftoff with which the subset ofinspection datapoints is associated; and (v) selecting the referencesignature from the signature library based at least in part on theliftoff.

In some embodiments, each inspection datapoint provides a respectivesensor impedance value.

In some embodiments, each inspection datapoint provides a respective atleast one property value associated with the test material. In someembodiments, each respective at least one property value is for anelectrical conductivity associated with the test material. In someembodiments, the feature is a notch. In some embodiments, the feature isa crack. In some embodiments, the feature is a scratch.

In some embodiments, the feature circumferential and axial location isdetermined.

In some embodiments, the subset of inspection datapoints comprises firstand second pluralities of inspection datapoints associated with firstand second sense channels of a sensor array, respectively, wherein thefirst and second sense channels are adjacent in the sensor array.

In some embodiments, the library of reference signature datapointsincludes at least two sets of reference signature datapoints. In someembodiments, the library of reference signature datapoints includesvalues for at least two test material layer thicknesses. In someembodiments, the library of reference signature datapoints includesvalues for at least two test material layer electrical conductivities.In some embodiments, the library of reference signature datapointsincludes values for at least two feature positions within the sensoractive area. In some embodiments, the library of reference signaturedatapoints includes values for at least two feature axial positionswithin a test material layer. In some embodiments, the library ofreference signature datapoints includes values for at least two sensorproximities to the hole material surface. In some embodiments, thelibrary of reference signature datapoints includes values for at leasttwo sensor axial positions within the hole.

The foregoing is a non-limiting summary of the invention, which isdefined by the attached claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1 is a block diagram a system for inspecting a test objectaccording to some embodiments;

FIG. 2 is a block diagram of a system for inspecting a hole in a testobject according to some embodiments;

FIG. 3A is a view of a mandrel according to some embodiments;

FIG. 3B is another view of the mandrel according to some embodiments;

FIG. 3C is a detailed view of the helical portion of the mandrelaccording to some embodiments;

FIG. 3D is a perspective view of a simplified mandrel to illustrateapplication of torque to the mandrel according to some embodiments;

FIG. 3E is another perspective view of the simplified mandrel toillustrate application of torque to the mandrel according to someembodiments;

FIG. 3F is a cut-away perspective view of the simplified mandrel toillustrate application of torque to the mandrel according to someembodiments;

FIG. 4A is an eddy current sensor array used as part of the mandrelsensor cartridge according to some embodiments;

FIG. 4B is a detailed view of the eddy current array portion of the eddycurrent sensor array according to some embodiments;

FIG. 5A is a flow diagram of a method for inspecting holes according tosome embodiments;

FIG. 5B is a chart showing 5 example scenarios of sense element positionrelative to various material layer interfaces;

FIG. 5C is a chart showing and addition 3 example scenarios of senseelement position relative to various material layer interfaces;

FIG. 6 is a flow diagram for a method of identifying material layersaccording to some embodiments;

FIG. 7 is a flow diagram for a method of generating a library ofsignature responses according to some embodiments;

FIGS. 8A, 8B, 8C, and 8D are various views and cut-always of a scannerfor controlling the mandrel according to some embodiments;

FIG. 9A is a perspective view of a hole test specimen formed frommultiple test layers according to some embodiments;

FIG. 9B is a cross-sectional perspective view illustrating various holefeatures according to some embodiments;

FIG. 9C is a perspective view of a hole test specimen formed with realcrack specimens according to some embodiments; and

FIG. 9D is a cross-sectional perspective view of a hole test specimenformed with real crack specimens according to some embodiments.

DETAILED DESCRIPTION

Recognizing the performance limitations of non-destructive evaluation(NDE) equipment is critical to any inspection application. The inventorshave recognized and appreciated that one of the challenges preventingeffective asset management is the high cost of effectively evaluatingNDE performance. A modular test specimen set is described thatsubstantially reduces the cost of providing a test set for an NDEperformance study. Specimens for such a performance study are made froma reconfigurable set of inspection plates. Each plate includes multipletest holes which are located symmetrically. The plates may be of variousthicknesses and materials. Each test hole may or may not have a featuresuch as a crack or machining notch. Such features may be located atvarious positions of the hole, such as at an edge, within the bore, andat various circumferential positions. A specimen is formed by stackingtwo or more plates and securing the stack together with an alignmenttool. A variety of specimens may be formed by using differentcombinations of inspection plates and flipping and rotating the memberplates.

In addition to the test set, a hole inspection system is disclosed aswell as an inspection procedure and data processing algorithm forperforming each hole inspection.

A scanner is described to which mandrels can be quickly connected andchanged enabling an inspector to quickly switch between differentmandrels (e.g., for different holes sizes and sensor configurations).

Non-destructive inspection of materials is critical for manyapplications, but such inspections can be challenging because of objectgeometry, material, and access issues. The inventors have recognized andappreciated the need for improved inspection procedures and equipmentwith which to perform such challenging inspections.

An example of a challenging inspection application is hole inspectionsuch as for holes used for bolts and other fasteners. For applicationsin which an unfilled hole can be inspected directly (e.g., without thefastener installed) there is an opportunity to provide a high resolutioninspection of the hole for a variety of features such as defects.

A system and method are provided for inspecting challenging materiallocations such as holes. The system may include a scanner and a sensorcartridge (“mandrel”) for inspecting holes with a sensor technology thatmay generally perform better where the sensor is in close proximity tothe wall of the hole, such as an eddy current sensor. The mandrel has ahelical portion to which such a sensor is attached. The radius of thehelical portion can be increased or decreased by applying a torque tothe helical portion thereby allowing the sensor to be inserted into ahole or pressed against the wall of the hole. Different mandrels may beprovided for different hole sizes and different sensor configurations.Also disclosed is an inspection procedure and data processing algorithmfor performing an inspection. The data processing algorithm utilizes asignature library for enhancing the detection or sizing of features ofinterest such as cracks. The algorithm and library can account formaterial edges, various material types.

The remainder of the Detailed Description is organized as follows.Section I provides an overview for an inspection system. Section IIprovides a description of a bolt hole inspection system. Section IIIprovides a procedure for inspecting bolt holes using the inspectionsystem. Section IV describes how data collected from the sensor for ahole may be analyzed to identify various material electrical andgeometric properties (e.g., number of layers, layer thickness, edgepositions, layer conductivities), and how various types of defects maybe detected. Section V describes how a signature library may begenerated and used for hole inspection, corrosion detection, and otherapplications. Section VI describes a modular test specimen set forholes. Finally, Section VII provides a closing discussion.

Section I—System Overview

FIG. 1 is a block diagram of a system 100 for inspecting a test object130. System 100 includes an instrument 110 and a sensor cartridge 140.Instrument 110 may be housed in a housing 107; in some embodiments thehousing is substantially cylindrical in shape. Sensor cartridge 140 mayhave a rigid connector which interfaces both mechanically andelectrically with an instrument side connector 105. Advantageously insome embodiments both the electrical and mechanical connections ofsensor cartridge 140 engage simultaneously with instrument sideconnector 105. In some other embodiments, sensor cartridge isfunctionally connected to instrument side connector 105 through a cable.Sensor cartridge 140 in some embodiments also includes a flexible sensor120, and a mechanical support 141 to which the sensor is attached.Sensor 120 may be attached to mechanical support 141 with glue, tape,double sided tape, or in any suitable way. Instrument 110 is configuredto provide excitation signals 121 to sensor 120 and measure theresulting response signals 123 of sensor 120. Response signals 123 maybe measured and processed to estimate properties of interest, such aselectromagnetic properties (e.g., electrical conductivity, permeability,and permittivity), geometric properties (e.g., layer thickness, sensorliftoff), material condition (e.g., fault/no fault, crack size, layer tolayer bond integrity, porosity, residual stress level, temperature), orany other suitable property or combination thereof including propertiesof the fabricated part and the powder. (Sensor liftoff is a distancebetween the sensor and the closest surface of the test object for whichthe sensor is sensitive to the test object's electrical properties.)

Instrument 110 may include a processor 111, a user interface 113, memory115, an impedance analyzer 117, and a network interface 119. Though, insome embodiments of instrument 110 may include other combinations ofcomponents. While instrument 110 is drawn with housing 107, it should beappreciated that instrument 110 may be physically realized as a singlemechanical enclosure; multiple, operably-connected mechanicalenclosures, or in any other suitable way. For example, in someembodiments it may be desired to provide certain components ofinstrument 110 as proximal to sensor 120 as practical, while othercomponents of instrument 110 may be located at greater distance fromsensor 120.

Processor 111 may be configured to control instrument 110 and may beoperatively connected to memory 115. Processor 111 may be any suitableprocessing device such as for example and not limitation, a centralprocessing unit (CPU), digital signal processor (DSP), controller,addressable controller, general or special purpose microprocessor,microcontroller, addressable microprocessor, programmable processor,programmable controller, dedicated processor, dedicated controller, orany suitable processing device. In some embodiments, processor 111comprises one or more processors, for example, processor 111 may havemultiple cores and/or be comprised of multiple microchips. Processing ofsensor data and other computations such as for control may be performedsequentially, in parallel, or by some other method or combination ofmethods.

Memory 115 may be integrated into processor 111 and/or may include“off-chip” memory that may be accessible to processor 111, for example,via a memory bus (not shown). Memory 115 may store software modules thatwhen executed by processor 111 perform desired functions. Memory 115 maybe any suitable type of non-transient computer—readable storage mediumsuch as, for example and not limitation, RAM, a nanotechnology-basedmemory, optical disks, volatile and non-volatile memory devices,magnetic tapes, flash memories, hard disk drive, circuit configurationsin Field Programmable Gate Arrays (FPGA), or other semiconductordevices, or other tangible, non-transient computer storage medium.

Instrument 110 may have one or more functional modules 109. Modules 109may operate to perform specific functions such as processing andanalyzing data. Modules 109 may be implemented in hardware, software, orany suitable combination thereof. Memory 115 of instrument 110 may storecomputer-executable software modules that contain computer-executableinstructions. For example, one or more of modules 109 may be stored ascomputer-executable code in memory 115. These modules may be read forexecution by processor 111. Though, this is just an illustrativeembodiment and other storage locations and execution means are possible.

Instrument 110 provides excitation signals for sensor 120 and measuresthe response signal from sensor 120 using impedance analyzer 117.Impedance analyzer 117 may contain a signal generator 112 for providingthe excitation signal to sensor 120. Signal generator 112 may provide asuitable voltage and/or current waveform for driving sensor 120. Forexample, signal generator 112 may provide a sinusoidal signal at one ormore selected frequencies, a pulse, a ramp, or any other suitablewaveform. Signal generator may provide digital or analog signals andinclude conversion from one mode to another.

Sense hardware 114 may comprise multiple sensing channels for processingmultiple sensing element responses in parallel. As there is generally aone to one correspondence between sense elements and instrumentationchannels these terms may be used interchangeably. It should beappreciated that care should be used, for example, when multiplexing isused to allow a single channel to measure multiple sense elements. Forsensors with a single drive and multiple sensing elements such as theMWM®-Array eddy current array available from JENTEK® Sensors, Inc., thesensing element response may be measured simultaneously at one ormultiple frequencies including simultaneous measurement of real andimaginary parts of the transimpedance. Though, other configurations maybe used. For example, sense hardware 114 may comprise multiplexinghardware to facilitate serial processing of the response of multiplesensing elements and for eddy current arrays other than MWM-Arraysmultiplexing may be used for combinations of sensing elements and driveelements. Some embodiments use MWM-Array formats to take advantage ofthe linear drive and the ability to maintain a consistent eddy currentpattern across the part using such a linear drive. Sense hardware 114may measure sensor transimpedance for one or more excitation signals aton one or more sense elements of sensor 120. It should be appreciatedthat while transimpedance (sometimes referred to simply as impedance),may be referred to as the sensor response, the way the sensor responseis represented is not critical and any suitable representation may beused. In some embodiments, the output of sense hardware 114 is storedalong with temporal information (e.g., a time stamp) to allow for latertemporal correlation of the data, and positional data correlation toassociate the sensor response with a particular location on test object130. Instrumentation may also operate in a pulsed mode with time gatesused to provide multiple sensing outputs and multiple channels used toacquire data from multiple sensing elements. If these sensing elementshave different drive-sense gaps (distance between a drive conductor (orelectrode) and the sensing winding (or electrode), then this is referredto as a segmented field sensor. Thus, sensor operation can be at asingle frequency, multiple frequencies, or in a pulsed mode where thedrive is turned on and off in a prescribed manner or switched betweentwo or more modes of excitation.

Sensor 120 is an eddy-current sensor; though in some other embodimentsit may be a dielectrometry sensor, thermography method, or utilize anyother suitable sensing technology or combination of sensingtechnologies. In some embodiments sensor 120 provides temperaturemeasurement, voltage amplitude measurement, strain sensing or othersuitable sensing modalities or combination of sensing modalities. Insome embodiments, sensor 120 is an eddy-current sensor such as an MWM,MWM-Rosette, or MWM-Array sensor available from JENTEK Sensors, Inc.,Marlborough, Mass. A discussion of some MWM-Array sensors is found inU.S. Pat. No. 6,784,662, issued on Aug. 31, 2004 which is herebyincorporated by reference in its entirety. Sensor 120 may be a magneticfield sensor or sensor array such as a magnetoresistive sensor (e.g.,MR-MWM-Array sensor available from JENTEK Sensors, Inc.), a segmentedfield MWM sensor, and the like. In some embodiments sensor 120 is aninterdigitated dielectrometry sensor or a segmented field dielectrometrysensor such as the IDED® sensors also available from JENTEK Sensors,Inc. Segmented field sensors have sensing elements at differentdistances from the drive winding or drive electrode to enableinterrogation of a material to different depths at the same drive inputfrequency. Sensor 120 may have a single or multiple sensing and driveelements. Sensor 120 may be scanned across, mounted on, or embedded intotest object 130.

In some embodiments, the computer-executable software modules mayinclude a sensor data processing module, that when executed, estimatesproperties of test object 130. The sensor data processing module mayutilize multi-dimensional precomputed databases that relate one or morefrequency transimpedance measurements to properties of test object 130to be estimated. The generation of suitable databases and theimplementation of suitable multivariate inverse methods are described,for example, in U.S. Pat. No. 7,467,057, issued on Dec. 16, 2008, andU.S. Pat. No. 8,050,883, issued on Nov. 1, 2011, both of which areherein incorporated by reference in their entirety. The sensor dataprocessing module may take the precomputed database and sensor data and,using a multivariate inverse method, estimate material properties forthe processed part or the powder. Though, the material properties may beestimated using any other analytical model, empirical model, database,look-up table, or other suitable technique or combination of techniques.

User interface 113 may include devices for interacting with a user.These devices may include, by way of example and not limitation, keypad,pointing device, camera, display, touch screen, audio input and audiooutput.

Network interface 119 may be any suitable combination of hardware andsoftware configured to communicate over a network. For example, networkinterface 119 may be implemented as a network interface driver and anetwork interface card (NIC). The network interface driver may beconfigured to receive instructions from other components of instrument110 to perform operations with the NIC. The NIC provides a wired and/orwireless connection to the network. The NIC is configured to generateand receive signals for communication over network. In some embodiments,instrument 110 is distributed among a plurality of networked computingdevices. Each computing device may have a network interface forcommunicating with other computing devices forming instrument 110.

In some embodiments, multiple instruments 110 are used together as partof system 100. Such systems may communicate via their respective networkinterfaces. In some embodiments, some components are shared among theinstruments. For example, a single computer may be used to control allinstruments. In one embodiment multiple areas on the test object arescanned using multiple sensors simultaneously or in an otherwisecoordinated fashion to use multiple instruments and multiple sensorarrays with multiple integrated connectors to inspect the test objectsurface faster or more conveniently.

Actuator 101 may be used to position sensor cartridge 140 with respectto test object 130 and ensure that the liftoff of the sensor 120 is in adesired range relative to the test object 130. Actuator 101 may be anelectric motor, pneumatic cylinder, hydraulic cylinder, or any othersuitable type or combination of types of actuators for facilitatingmovement of sensor cartridge 140 with respect to test object 130.Actuators 101 may be controlled by motion controller 118. Motioncontroller 118 may control sensor cartridge 140 to move sensor 120relative to test object 130.

Regardless of whether motion is controlled by motion controller 118 ordirectly by the operator, position encoder 103 and motion recorder 116may be used to record the relative positions of sensor 120 and testobject 130. This position information may be recorded with impedancemeasurements obtained by impedance instrument 117 so that the impedancedata may be spatially registered.

For some applications the performance of system 100 depends (among otherthings) on the proximity of sensor 120 to test object 130; that is tosay the sensor liftoff may be critical to performance for suchapplications. For example, crack detection in an aerospace applicationmay require cracks 0.5 mm (0.02 inches) in length be reliably detectablein test object 130 (e.g., a turbine disk slot). In order to achievereliable detection of a small crack, sensor 120's liftoff may need to bekept to under 0.25 mm (0.010 inches). Further, for such an application,sensor 120 may preferably be a sensor array, thus the liftoff of eachelement in the array may need to be kept to under 0.25 mm (0.010inches). (It should be appreciated that these dimensions areillustrative and the specific requirements will be dictated by thedetails of the application.) Measurements may be complicated when testobject 130 has a complex curved surface that may change along ameasurement scan path.

Section II—Bolt Hole Inspection System

Bolt holes in aerospace and other fields may require inspection todetermine if the hole has fatigue cracks or other forms of damage. FIG.2 shows a system 200 for hole inspection according to some embodiments.System 200 may generally be considered an embodiment of system 100, andthus the discussion of system 100 is generally applicable to system 200.System 200 may include an instrument 110 such as that described withreference to FIG. 1, a handheld scanner 210, and a sensor cartridge 140.

FIG. 2 shows an example test object 130 having layers 131-A, 131-B, and131-C, generally referred to as layers 131. Test object 130 may have anynumber of layers 131 and the illustration of three layers is for exampleonly. Test object 130 has a hole 132 to be inspected. In somediscussions the term “stackup” is used to refer to the sequence oflayers in a test object 130.

Scanner 210 is a device for changing the position of sensor cartridge140 relative to hole 132 and facilitating inspection measurements of thehole by instrument 110. Scanner 210 may be connected to instrument 110by a cable such that instrument 110 can be a few feet away from scanner210 or integrated into a single unit with instrument 110. Though,instrument 110 may interface with scanner 210 and sensor cartridge 140in any suitable way.

Scanner 210 may control both the axial and circumferential position ofsensor 120 within hole 132, though in some embodiments sensor 120 may bean array of sufficient size such that scanning in only one direction isnecessary. For example, sensor 120 may be a sensing array that providescoverage around the entire circumference of hole 132 such that scanningmay only need to be performed in the axial direction. As anotherexample, sensor 120 may be a sensor array that provides coverage alongthe entire depth of the hole such that scanner 210 may only need to scanin the circumferential direction. In some other embodiments, sensor 120only provides partial coverage in both the axial and circumferentialdirections and scanner 210 may control motion in both directions tofacilitate inspection of the hole.

Scanner 210 may include circumferential actuator 201, circumferentialposition encoder 203, axial actuator 202, axial position encoder 204 andtrigger 205. Scanner 210 also includes hardware for mechanicallysupporting sensor cartridge 140.

Circumferential actuator 201 may be used to control the circumferentialposition of sensor 120 relative to hole 132. Similarly, axial actuator202 may be used to control the axial position of sensor 120 relative tohole 132. Any suitable actuator may be used such as an electric motor.In some embodiments the axial and/or the circumferential position may becontrolled by hand or using a mechanical device without use ofautomation.

Circumferential position encoder 203 and axial position encoder 204record the position of sensor 120 in the circumferential and axialdirections, respectively. In some embodiments, the circumferentialand/or axial position encoders may not be required.

Trigger 205 provides a mechanism for user input to control the scanningand inspection process. Trigger 205 may be, for example, a button.Though trigger 205 may generally be any suitable form of user input suchas those described in connection with user interface 113 (FIG. 1).

An example of sensor cartridge 140 is shown in FIG. 3A as sensorcartridge 300. Sensor cartridge 300 may be used, for example, withsystem 200, though sensor cartridge 300 may be used in any suitable way.Sensor Cartridge 300 includes a mechanical support and a sensor 320.Sensor cartridge 300 may be used for inspecting bolt holes or other holelocations as discussed further herein. Sensor cartridge 300 has anexpansion element 310 which includes a helical portion to which a sensorarray portion 321 of sensor 320 is mounted. A shaft 340 runs the lengthof cartridge 300 and down the center of helical portion 310. The distalend of shaft 340 is operably fixed to the distal end of the expansionelement 310. The proximal end of shaft 340 has a nut 341 operably fixedthereto. Sensor Cartridge 300 may be used, in part, by rotating shaft340 relative to expansion portion 310. This relative motion will causethe diameter of the helical portion of expansion element 310 to expandor contract, depending on the direction of the relative motion. Forexample, sensor cartridge 300 may be inserted into a hole in acontracted state and used to scan the hole in an expanded state wherebysensor array portion 321 is in intimate contact with the wall of thehole achieving a low liftoff.

The term “operably fixed” as used herein is used to describe amechanical relationship between two components wherein the twocomponents are connected in such a way that a relative movement betweenthe two components requires one or both of the components to flex (orthe component therebetween, if any, to flex). The two components may befixed to one another mechanically (e.g., splines, adhesives), the twocomponents may share a piece of material (i.e., the two components areeffectively portions of a single component), or the two components maybe fixed to one another through one or more additional components.

A description of some further embodiments of sensor cartridge 300 arenow provided with reference to FIGS. 3A-F.

Sensor cartridge 300 according to some embodiments is shown in FIG. 3A.Sensor cartridge 300 may include a sensor 320. Sensor 320 has a sensorarray portion 321 which has the elements for obtaining sensor data.Sensor array 320 may be an eddy current array sensor such as sensor 400shown in FIG. 4A. Sensor 320 may have a lead portion 322 that providesand connection between the sensor array portion 321 and the measurementinstrumentation (e.g., instrument 110). FIG. 3B shows another view ofsensor cartridge 300 without sensor 320 to further illustrate someadditional aspects.

Sensor cartridge 300 may have a control wheel 303 that providescircumferential motion of the sensor cartridge 300 relative to thehandheld scanner 210 and/or the test object 130. In some embodimentscontrol wheel 303 may not be included as part of sensor cartridge 300;for example, if sensor 320 provides full circumferential coverage ofhole 132 it may not be necessary to scan circumferentially. Though, insome such embodiments control wheel 303 may be included as a mechanismwith which to apply torque to the helical portion and thus expand orcontract the radius of the helical portion. Examples of control wheel303 include a gear, a pulley, and a timing pulley. In these cases, thecontrol wheel 303 would interface with the circumferential actuator 201.Alternatively, control wheel 303 can be a functional component ofcircumferential actuator 201. For example, circumferential actuator 201could be the stator of an electric motor and control wheel 303 could bethe rotor of an electric motor and the electric motor is only completewhen the two components are combined.

Sensor cartridge 300 has expansion element 310. Expansion element 310may have a helical portion having one or more helixes coaxial with shaft340. In the embodiment shown in FIG. 3A, the helical portion has a firsthelix 311 and a second helix 312. The helix can twist in eitherdirection (clockwise or counterclockwise). Helix 311 is shown wider (inthe axial direction) than helix 312. Helix 311 is wide enough where thesensor array portion 321 of sensor 320 mounts to it. Helix 311 may alsoinclude a recess 319 in the surface such that sensor 320 may be mountedinside of the recess as shown in FIG. 3C. (Note shaft 340 is not shownfor clarity.) The angle of helix 311 and 312 should be the same, butvarious helix angles (or turns per unit distance) may be used. In someembodiments it is desirable to orient a sensor at a 45 degree anglerelative (though any suitable orientation may be used). The helix angleand/or the orientation of recess 319 may be selected to facilitatesecuring sensor 320 at the desired sensor orientation.

A simplified schematic showing the application of torque to the outercomponent is shown in FIGS. 3D-3F where the outer expansion element islabeled 350, and the rotating shaft is labeled 360.

Expansion element 310 may be made of a semi-rigid material such thatrelative circumferential motion between the proximal and distal ends ofthe expansion element 310 allow expansion element 310 to increase and/ordecrease in diameter without causing deformation of the material. Thechange in diameter must be sufficient to allow for a contracted statewhere the diameter of the expansion element 310 is less than diameter ofhole 132 and an expanded state where the diameter of the expansionelement 310 is equal to or greater than the diameter of hole 132. Whenthe expansion element is in hole 132 and placed in the expanded state,the diameter of the expansion element must be able to be restricted tothe diameter of hole 132 without causing deformation of the material.

In one embodiment, the expansion element 310 is in the expanded statewhen undeformed and the contracted state is achieved by moving thedistal end relative to the proximal end. In a second embodiment, theexpansion element 310 is in the contracted state when undeformed and theexpanded state is achieved by moving the distal end relative to theproximal end. The relative motion may be rotation or displacement orboth.

Sensor cartridge 300 has a shaft 340 that may extend the length of thecartridge. Shaft 340 may be made of metal, or any suitable material andmay have a cylindrical or other cross section. The distal end of shaft320 may be operably connected to the distal end of expansion element310. In some embodiments, shaft 340 has a key at its distal end whichengages with the distal end of expansion element 310. The key may beoperably fixed to the distal end of the expansion element, or it mayhave a loose fit such that some rotation of the shaft relative to theexpansion element is permitted before the key is engaged and torque istransferred between the shaft and expansion elements.

Sensor cartridge 300 may have a reel 305 about which the lead portion322 of sensor 320 may be wrapped. More specifically, during operation,as sensor cartridge 300 is rotated about its axis to scan a hole sensorlead portion 322 may wind up or unwind from reel 305. Alternatively,reel 305 may contain sliding electrical contacts such that the leadportion 322 may remain stationary while the sensor array portion 321rotates. In some embodiments a slip ring is used to electrically connectto sensor 320 eliminating the need for reel 305. To support use of aslip ring sensor cartridge 300 may have sliding electrical contacts towhich sensor 320 is connected. A slip ring on the scanner may beconfigured to connect electrically with the sliding electrical contactsfor sensor excitation and measurement.

Sensor cartridge 300 may further include a circular disc 304 between thecontrol wheel 303 and the reel 305. Circular disc 304 may separate reel305 and control wheel 303 such that the lead portion 322 of the sensordoes not interfere with the control wheel during operation. Circulardisc 320 may connected to reel 305 by a flared portion which tends toforce the lead portion 322 towards the reel 305 as the lead portion iswound up on the reel.

Sensor cartridge 300 has a bearing 306 positioned between reel 305 andthe expansion element 306. Bearing 306 may be a ball bearing but anysuitable bearing may be used. An inner ring of bearing 306 may be fitaround reel 305 or expansion element 310 at a suitable location. In someembodiment the inner ring of bearing 306 is fixed at this location whilein some other embodiments the fit is loose.

Sensor cartridge 300 may have circular mount 302 located along its axis.Circular mount 302 may be configured to slide within a bearing on ascanner when sensor cartridge 300 is installed for use.

Sensor cartridge 300 has a nut 341 connected to the proximal end ofshaft 340. Nut 341 is any suitable mechanism by which the scanner canallow a torque to be exerted on shaft 340. Nut may be a hexagonal nut,keyed in any suitable way, or circular. Alternatively, nut 341 can beomitted if shaft 340 can be actuated directly.

Sensor cartridge 300 may connect to a scanner for operation such ashandheld scanner 210 of FIG. 2. In the embodiment shown in FIG. 3A, thescanner provides the circumferential actuator 201 that interfaces withthe control wheel 303 to turn the sensor cartridge 300. The nut 341 cannormally spin freely, but can be selectively constrained. Thisconstraint prevents the rotation of nut 341, shaft 340, and the distalend of expansion element 310. In this condition, the rotation of thecontrol wheel causes rotation of the proximal end of expansion element310, but not the distal end of expansion element 310 resulting in thecontraction (or expansion depending on the direction of the torque andhandedness of the helical portion) of the radius of the helical portion.

Sensor 400 shown in FIG. 4A is an eddy current array sensor. Sensor 400includes a connector portion 410, a lead portion 420, and an eddycurrent array portion 430. A detail of the eddy current array portion430 is shown in FIG. 4B. Sensor 400 has a drive winding 431 with alinear portion, seven sensing elements 432, and dummy elements 433flanking the sensing elements. Sensing elements 432 and the drivewinding 431 have leads in lead portion 420 that connect to the connectorportion 410 so that sensor 400 may be excited and measured by suitableinstrumentation. It should be appreciated that the design of sensor 400is merely and example, and that different sensor types and sensorgeometries may be used in accordance with the needs of the specificapplication.

FIGS. 8A-8D show various views and cut-always of a scanner 800 accordingto some embodiments. Scanner 800 may be used with sensor cartridge 140in ways discussed in connection with systems 100 and 200. In someembodiments scanner 800 is used with sensor cartridge 300. For example,an inspection kit may include a variety of sensor cartridges ofdifferent diameters (e.g., for different hole sizes), different axiallengths, and/or different sensor types or sensor geometries. Scanner 800may allow for the easy and quick interchange of such different sensorcartridges. FIG. 8A shows a perspective view of scanner 800. FIGS. 8B-8Dshow a cutaway view so that various components of scanner 800 arevisible and their function readily apparent.

Section III—Bolt Hole Inspection Procedure

FIG. 5A shows a flow diagram of a method 500 for inspecting holes.Method 500 may be performed using the inspection system described inconnection with FIGS. 1 and 2, though method 500 may be performed in anysuitable way.

At step 501 method 500 receives basic input about this inspection to beperformed. This information may be provided, for example, by aninspector responsible for performing the inspection. This informationmay be used to support record keeping for the inspection and mayincorporate location specific information about the stackup. Thislocation specific information could help to simplify the inspection,such as the expected layer thicknesses and material layer types, but itis not required for the inspection. This information could be used toverify that the material stackup (material layer thicknesses and type)are consistent with the output of the inspection procedure.

At step 502 method 500 acquires scan inspection data within the hole. Aspart of step 502 the inspector may insert an inspection probe into thebolt hole and obtain one or more sets of scan data around thecircumference of the hole. This could be done with a single plungeposition as the circumferential position is varied, then increasing theplunge as needed and repeating the circumferential scans until theentire thickness of the hole is inspected. Note that the sense elementpositions relative to the edges or material layer gaps must beconsistent enough and not change too rapidly to distort flaw responsescompared to signature responses. What is “consistent enough” will dependupon the detection requirements for a specific inspection application. Averification check may be performed on the data to ensure that the axialposition of the sense elements do not change too rapidly relative to theedge since this could distort any potential flaw responses.

At step 503 method 500 assesses the measurement data to identify avariety of features in the material stackup. For example, one featurecould be the material type for each layer, such as identifying eachlayer as an aluminum alloy or a titanium alloy, and this could bedetermined by the nominal conductivity of the material within a layer.Another example is the thickness of each layer, which could be assessedthrough the axial variation of the conductivity. Another example is todetermine the sense element position relative to the edges of thematerial layer as this could affect the signature responses selected forthe shape filtering approach. FIGS. 5B and 5C show schematic diagrams ofseveral potential hole inspection scenarios for aluminum and titaniummaterial layers and the expected response to be associated with eachsense element for an exemplary seven-channel eddy current sensor array.The seven sensing channels are numbered 1 through 7 and the sensor shownis sensor 400 from FIGS. 4A and 4B. Step 503 allows identification ofwhich channels are only over metal without an interface, which channelsare near an interface, which channels are over an edge (metal/airinterface) or an internal interface (e.g., metal/metal interfacepotentially with a gap between the metal layers), and which channels arecompletely off of the metal (e.g., only in air).

At step 504 method 500 estimates the sense element locations relative tothe edge. This provides a more quantitative estimate of the locationcompared to Step 503, which was binary and simply indicated either on oroff of an interface. The lift-off and the responses from multiple senseelements can be used to provide this estimate of the location. Thisallows the appropriate signature responses to be selected from asignature library and could allow for corrections of property estimates(such as the electrical conductivity) for channels that are near but notyet over a material interface. This may be a local correction sincethere are usually circumferential variations in the sense elementresponse due to the mechanical misalignment of the sensor array with thehole axis. Step 504 may also include a verification check to ensure thatthe circumferential variation of the channel responses are withinappropriate ranges. Note that the actual estimation of the sense elementlocation relative to material edges may not be needed if enoughcharacteristic flaw response shapes are included in the library ofsignature responses to capture the sensor flaw response variation withproximity to a material edge.

Method 600 discussed in connection with FIG. 6 provides examples of howaspects of steps 503 and 504 may be implemented according to someembodiments.

At step 505, method 500 performs data analysis to identify any defectswithin the body of the hole including at edges and at internal materialinterfaces. Cracks are an example defect, though any suitable type ofdefect may be targeted for detection. In some embodiments a shape filteralgorithm is used to locate responses within the scan data that aresimilar to characteristic shape responses from a stored library ofresponses. This can be used to highlight and reveal the defect responsewithin noisy inspection data. Shape filtering may be implemented, forexample, in ways discussed in Goldfine II, though any suitable shapefiltering methodology may be used.

The sensor responses to a geometric feature may vary with thecircumferential position of the MWM-Array relative to the geometricfeature. As a result, the shape filtering algorithm may use signatureresponses that span a range of circumferential positions. In this waythe algorithm can detect such geometric features within the scan dataand also determine which signature within the library is the best matchfor each of the features detected and for each of the channels withinthe array. The best-match signatures may then be identified in thefiltered data.

In some embodiments the identification information from the previoussteps in method 500 are used to down-select the appropriate signatureresponses to be applied with the shape filter. This may reduce theprocessing time or processing requirements by only considering a subsetof signatures from the signature library. Since the processing time forthe shape filter algorithm may scale with the number of signatures, theprocessing speed can be improved when only a subset of the entiresignature library is needed. Though, in some embodiments all signaturesin the library may be processed.

The signature library may be initially generated prior to theinspection. Signatures in the library may cover the range of variationslikely to be observed, such as the sense element position relative tothe edge, defect type, defect positions relative to internal andexternal interfaces, and material type (e.g., titanium, aluminum, ormore generally based on the properties of the material affecting thesensor response such as electrical conductivity). Note that one or morecharacteristic responses can be added to the library for known orverified defect conditions during an inspection by extracting therelevant portion of the inspection data and storing it in the library ofresponses. The output of the algorithm is then a filtered response foreach sense element channel, which can be visualized as a B-scan plot foreach channel or a C-scan image of a concatenated version of the channelresponses for a scan set. Further discussion of generating and updatinga signature library are discussed in Section V, below.

In some embodiments multiple scans are required to image the entireinternal surface of the hole. These scans may be processed sequentiallyas the data is collected, in parallel, combined into a dataset analyzedfor analysis or in any suitable way. In the embodiment of method 500shown in FIG. 5A, it is contemplated that multiple scans may be takenand processed sequentially. At step 506, method 500 determines ifadditional data is to be analyzed, and if so returns to repeat steps503, 504, and 505. These additional scans may be performed with thesensor array plunged further into the hole with some overlap with theprevious scan(s) to provide complete coverage. All of the scan datacould be obtained during step 502. In terms of the additional processingfor all of the sets of data, this could involve aligning the startingcircumferential position of each in order to create a composite scanimage of the estimated properties, or filtered properties, for theinspection. This alignment may be needed since manual scanning mayrequire individual adjustments for each scan set (e.g., axial andcircumferential offset for each scan). This processing could alsoinclude an autonormalization process, which uses a region of relativelyuniform property data for each channel that is unlikely to include aflaw and adjusting the values to reduce channel-to-channel variation.The output of this analysis may be a composite C-scan image of theinspection results that can be used to identify likely crack locations.It is generally also desirable for the image to highlight the materialtype of each material layer as well as any material layer edges,including internal material interfaces.

Section IV—Layer Thickness and Material Identification

FIG. 6 is a flow diagram of a method 600 for processing scan data.Method 600 may be used, for example, to determine the layerconfiguration of a bolt hole. In some embodiments method 600 isimplemented as a module, such as modules 109 in FIG. 1. Though, method600 may be implemented in any suitable way. In some embodiments, method600 is used to process material property data collected by scanning theinner surface of a bolt hole in order to determine the material type andthickness of the different layers comprising the bolt hole geometry.Method 600 may be used to determine if there are variations of thisgeometry in the circumferential and axial directions. While method 600is discussed with reference to bolt holes, it should be appreciated thatit may be used for any suitable hole or application.

At step 601 material property data such as electrical conductivity andsensor liftoff is collected by scanning the inner surface of a bolt holewith a sensor consisting of an array of sense elements. For example,immittance measurements may first be collected from the sensor and thenprocessed in a suitable way to provide material property estimates. Insome embodiments a multivariate inverse method is used in combinationwith a database of precomputed sensor responses to estimate materialproperties.

The sensor may be scanned in the circumferential direction with thesensing elements distributed at least partially in the axial directionsuch that the different sense elements provide separate measurements inthe axial direction producing two-dimensional material property imagesof the inner surface of the bolt hole. Several overlapping scans,shifted in the axial direction relative to each other, may be necessaryto cover the entire inner surface of the bolt hole. The scans may beperformed such that for the first scan one or more sense elements willbe above the top of the hole and in air and for the last scan one ormore sense elements will be below the bottom of the hole and in air.

Method 600 processes the data in successive (possibly overlapping)angular intervals. The size of these intervals may be an adjustableparameter of the module in which method 600 is implemented. Such aparameter may be set in accordance with the expected level of variationof the layer configuration around the circumference of the bolt holeinner surface.

At step 602 data in a first angular interval is extracted forprocessing. (As method 600 repeats, the angular interval is incrementedaccordingly.)

At step 603 the material property data (e.g., electrical conductivity)within the angular interval is averaged separately for each senseelement and each axially incremented scan. If for example, the sensorarray had seven sensing elements and five different axial positions werescanned, step 603 would produce 35 averages for each material property.

At step 604 the averaged data is searched in the axial direction lookingfor a transition from material properties consistent with a senseelement in air to those of a material layer. This identifies the topedge of the first layer of the bolt hole geometry in this angularinterval.

At step 605 the algorithm uses the estimated material properties toidentify the material type of the layer. In some embodiments, a set ofmaterial types with material properties distinguishable using the sensormeasurements is known such that the layer can be identified as amaterial type in the set. Consider for example, if only an aluminumalloy and a titanium alloy are expected to form the layers of the bolthole. Method 600 may be able to identify a layer as the aluminum alloyor the titanium alloy based on the nominal electrical conductivity ofthe material layer.

At step 606 the axial extent (thickness) of the layer is determined andthe bottom edge of the layer is located. If adjacent layers aredifferent materials with sufficiently distinct material properties thenthe method used in this step can consist of searching for such atransition of the material properties. In cases where adjacent layersmay be composed of the same or similar materials then the method forlocating the bottom edge of the current layer is to use the shape of thevariation of the effective material properties in the axial direction.For example, the edge effects on the effective material properties canresult in a local maximum or minimum of one of the material propertiesat the axial midpoint of the layer.

At step 607, method 600 branches depending on whether or not the bottomedge of the layer located at step 606 marked a transition to airindicating that the bottom of the hole has been found. In the case of atransition to air, method 600 advances to step 608. Otherwise there is anew layer to analyze and the algorithm loops back to step 605.

At Step 608, method 600 has completed the analysis of the currentangular interval and branches depending on whether or not there areadditional angular intervals to process. If there are unprocessedangular intervals method 600 returns to step 602 to begin processing thenext angular interval. In some embodiments, some or all angularintervals are processed in parallel to expedite method 600.

If there are not unprocessed angular intervals analysis of the layerconfiguration is complete and method 600 advances to step 609.

At Step 609, method 600 displays a graphical representation of the layerconfiguration of the inner surface of the bolt hole. This may consist ofa two-dimensional image of the inner surface showing different materialtypes in different colors and with layer boundaries delineated. In someembodiments, step 609 is part of an automated reporting process whichmay also include reporting the the location of primary and secondaryfeatures as discussed in Section V.

Estimation of layer thickness is one innovation that is improved byusing an eddy current sensor with a linear drive and two or more sensingelements, where the data for each sensing element is recordedsimultaneously. Though, method 600 may be used in combination with anysuitable sensor type or configuration.

Section V—Shape Filtering

As discussed earlier in Section III, shape filtering may be applied toimprove detectability of defects in holes or other test objects moregenerally. A shape filter module is described that may be used toenhance sensitivity to a primary feature and/or to reduce the responseto a secondary feature. In this context a “primary feature” is a featurethat is the target of the inspection. A primary feature may be amaterial defect such as a crack, a material gap indicative of corrosion,or any other feature which an inspection is designed to detect. A“secondary feature” may be described as benign feature that has acharacteristic sensor response. For a bolt hole inspection a secondaryfeature may be, for example, a scratch, the presence of debris betweenlayers, response to a layer edge, dings at a corner, a burr or otherdebris from cleaning the hole, out of roundness effects or other surfaceor geometric features. For corrosion imaging in a joint a secondaryfeature could be, for example, a fastener response, the response of agroove between materials in a layer, or a response of a material edge.Reducing the response of a secondary feature, that is inconsequential,may allow a primary feature to be more readily detected or accuratelysized. A shape filter module may, for example, be implemented in any ofthe ways described in connection with modules 109 (FIG. 1).

For both primary and secondary features, a goal of the shape filter isto locate responses within the scan data that are similar tocharacteristic shape responses from a stored library of responses. Thisstored library of responses can be described as a signature library. Thesignature library can be used to reveal and enhance the primary (e.g.,flaw) response within inspection data that includes noise or other errorsources or responses from secondary features. The sensor responses to ageometric feature vary with the properties of the feature. For example,in the case of a crack the sensor response may vary with crack lengthand depth (and similarly for an EDM notch which may be used to simulatea crack). Similarly, in the case of a secondary feature, the sensorresponse varies with material type and dimensions of the secondaryfeature (e.g., the sensor response varies with the diameter of afastener head for corrosion imaging, or the thickness of a layer or ascratch for a bolt hole inspection). The sensor responses can also varywith the sensor position relative to the feature. As a result, the shapefilter module may use signature responses that span a range ofproperties that capture the variability of the feature response so thatits response can be enhanced for detection or reduced for anomalysuppression, as appropriate. In some embodiments, a material property ordimension may be estimated and used to select the appropriate signaturefrom the library. This property or dimension might be, for example, theconductivity of a layer, the distance from an edge, the thickness of alayer, or the position of the defect response between adjacent channelsin a sensor array. A convenient method for applying this filter is todetermine which signature within the library is the best match for eachof the features of interest and for each of the channels within thesensor array. The best-match signature is then used to highlight thefeature in the filtered data for feature enhancement or used to removethe feature response for secondary feature (anomaly) suppression.

FIG. 7 provides a method 700 for building a signature library accordingto some embodiments. A goal of method 700 is to have a small number ofsignatures which enables faster processing for the filtering algorithmwhile still detecting and performing appropriate analysis on allfeatures of interest. The steps of method 700 are detailed and exampleapplications are discussed. Method 700 may be used for building asignature library for primary features (e.g., for feature detection,such as crack detection in bolt holes) and secondary features (e.g., foranomaly suppression, such as the removal of fastener responses toimprove detection and characterization capabilities for corrosionmaterial loss).

At step 701 method 700 sets up the inspection system. This involvesassembling the components for the system, performing the calibration orstandardization to obtain repeatable and reproducible values for knownmaterial conditions, and verifying the performance of the system. Theinspection system may be an embodiment of system 100 (FIG. 1), thoughany suitable inspection system may be used. This performanceverification can be performed as electrical conductivity or magneticpermeability measurements on reference materials with known propertiesor on unflawed materials with known nominal properties. Typically thisverification confirms that the property value measured with the systemis comparable to the value expected for the material. The performanceverification may also include variations in the liftoff. Note that theliftoff test is aimed at confirming that the measured property value isessentially independent from the liftoff over the liftoff range ofinterest for the inspection. Step 701 may also include loading, orproviding access to, a previously obtained library of signatureresponses.

At step 702, method 700 acquires data from a test object. This step mayinclude a verification that the scanned area fully covers the region ofinterest for the inspection and an initial identification of observablefeatures in the measured scan responses. This could also include thedetermination of categorization of these features as relevant, such as acrack or material loss, or anomalous (or non-relevant), such as aboundary between two material layers or a fastener, but this is only aninitial determination. In some embodiments, step 702 involves insertinga probe (e.g., sensor cartridge 300, FIG. 3A) into a bolt hole andobtaining one or more sets of scan data around the circumference of thehole. This could be done with a single plunge position as thecircumferential position is varied, then increasing the axial plunge asneeded and repeating the circumferential scans until the entirethickness of the bolt hole is inspected. As another example, step 702may involve one or more scans over the skin surface of material havingsubsurface structural elements, such as a lap joint. After the scan overthe structural elements, the fastener pattern and possibly the layout ofthe subsurface structural elements should be visible. Generally thefastener responses would be considered anomalous compared to thematerial loss from corrosion that could be in the vicinity of thefasteners.

Step 703 refers to the application of a processing algorithm todetermine the layer configuration. This is done to identify differentmaterial types and layer thicknesses within the test material. Thiscould be used to verify the inspection as well, if apriori knowledge ofthe material stack up for the region of interest is available. Forexample, for bolt hole inspection a single excitation frequency could beused with the measured responses at different axial positions within thebolt hole to determine the material type and thickness of each of thedifferent layers comprising the test material geometry. Similarly, for alap joint, one or more frequency measurements could be used to identifythe material type and thickness of the layers perpendicular to theinspection surface. In particular, high frequency measurements, wherethe penetration depth is comparable to or smaller than the thickness ofthe near surface skin layer of the test material, can be used toidentify the type of material, such as an aluminum alloy, for the skin,to determine the presence of the near surface defects such as pits, andto determine regions of nonconductive coating variations, such as paint,that may be of interest. In one such embodiment the thickness of thepaint (estimated from the liftoff) or an estimated layer thickness maybe used to select the appropriate signatures for defect detection orsecondary feature suppression. Similarly, low frequency measurements,where the penetration depth is comparably larger than the thickness ofthe near surface skin layer of the test material, can be used incombination with the high frequency measurements to determine thethickness of the skin layer and to identify the type of material for thesubsurface layers, such as an aluminum alloy or a magnetic steel. Thisstep could also be used to determine the presence of the subsurfacedefects such a corrosion material loss but it is preferable to performthis evaluation after the anomalous responses have been removed, for theexample of corrosion imaging. Method 600 discussed above in connectionwith FIG. 9 provides an example embodiment of how step 703 may beimplemented.

At step 704 method 700 determines the filter response for the inspectiondata for a library of signature responses. Subsets of the signaturelibrary responses could be used. For example, signature responsesappropriate for cracks in titanium alloys or aluminum alloys could beused along with the layer type identification of step 703 to reduce thenumber of signatures used as part of the shape filtering approach sincethe signatures for the incorrect material type may not be inappropriate.In other words (and for example), using crack signatures for a titaniumalloy may not be appropriate for an aluminum alloy. For the bolt holeinspection, the filtering process is aimed at improving the response tothe defect features. However, for the material loss inspection, thefiltering process is aimed at the removal of anomalous responses notassociated with the material loss such as the responses from fastenersor material edges. In this case, different signature responses may berequired based upon the material type of the subsurface layers, such asa magnetic steel alloy versus an aluminum alloy. Note in one embodimentthe signatures may be extracted and placed in the signature library fromneighboring fasteners. In one such embodiment, after the detectionprocess is first completed, if the fasteners that are observed by theoperator have not been completely removed (suppressed) from the responseimages, then the operator may extract a response from a apparentlysimilar fastener (based on his visual observations or on the sensorresponses) in the neighborhood of the joint area of interest. This wouldenable improved fastener suppression (or suppression of other suchsecondary features) by using similar features in the neighborhood oreven within the inspection area of interest. In another embodiment,instead of the immediate neighborhood, a signature might be extractedand added to the library for secondary feature suppression, from asimilar area on the same aircraft or even from a different aircraft withsimilar or the same fastener types and joint configurations.

At step 705 method 700 evaluates the filtered responses with a goal ofdetermining the adequacy of the filtering process in Step 704. Thisevaluation can take a variety of different forms. For defect detection,such as crack detection or material loss detection, this could be acomparison to known defect feature conditions. The comparison could be ahit/miss type of analysis to determine if known defects are missed or acomparison of measured response levels to determine if the responselevel correlates with the properties of the defect condition, such ascrack length, crack depth, or material loss depth. For anomalysuppression, the evaluation of the filtered response in the vicinity ofeach anomaly can be assessed through the variation of the impedanceresponse and/or a measured property response (such as material layerconductivity or thickness). This evaluation could simply determine ifthe response variation is within a threshold level of the baselinevalue. For example, the electrical conductivity typically varies byseveral % IACS in the vicinity of typical fasteners within joinedaluminum alloy panels having a nominal electrical conductivity of 32%IACS. The threshold could be that the electrical conductivity differencebetween the fastener response and the panel response must be less than0.5% IACS. If the anomaly response exceeds the threshold then thisresponse can be a candidate for a signature to be added to the libraryof signature responses.

Step 706 provides a decision opportunity based upon the adequacy of thefiltering process. As with Step 705, this decision can take a variety ofdifferent forms. For defect detection such as a bolt hole inspection,this could include an assessment of the probability of detection, thefalse call rate, and generation of a receiver operator curve (i.e., PODvs False call rate), or another form of reliability and repeatabilitystatistical analysis to independently assess feature detection andsuppression performance. For anomaly suppression, this assessment isprimarily aimed at determining if the features of interest, such asmaterial loss from corrosion or a crack in a bolt hole, can be detectedeven if the anomalies are not fully or completely suppressed. Forexample, if a gap between the joint skin layers leads to an anomalyresponse that is not suppressed, this may not affect the detectabilityof the material loss if the areal extent of the material loss ofinterest is much larger than a dimension of the gap response. However,if any of the known anomaly responses, such as fastener responses,exceed a threshold response, then the anomaly suppression may not beadequate. If the performance level is satisfactory, then the procedurecan proceed to Step 709 and the results of the measurement or theinspection can be reported. In either case, if the performance level isnot satisfactory, then the signature library should be updated as inStep 707.

Step 707 refers to actions performed to update the signature library andto improve performance for the measurement or inspection. In eithercase, for defect detection or anomaly suppression, it may be desirableto remove signature responses from the library in order to reduceprocessing time with the shape filter. The signatures to be removed maybe inappropriate for the inspection, such as the fastener type or sizemay not be relevant to a particular inspection or the signatures may besimilar to other signatures in the library, or the signatures maycorrespond to feature dimensions that are not of interest, such as 0.010in. long cracks when only cracks that are longer than or equal to 0.020in. are of interest. Also for both defect detection and anomalysuppression, the response from a feature of interest can be captured andadded to the library of signature responses. This could be from datathat was acquired as part of Step 702 and could be for a crack that wasmissed as part of the inspection or for a fastener that had a responsewhich was not suppressed. In the case of bolt hole inspection,signatures might be acquired from representative samples withrepresentative features that must be not only suppressed but alsoidentified. Features in bolt holes such as scratches, debris, shallowpits, burrs, or other surface and geometric anomalies might need to beboth suppressed and identified. In some cases the identification of ananomaly such as a burr or debris, might result in adjustment of theprimary feature detection threshold to avoid a false call. In anothersuch embodiment the signature of the inconsequential anomaly (e.g. aburr) might be used to suppress the associated response, withoutremoving the response of a primary feature that might occur in thevicinity. In one embodiment, the identification of the inconsequentialfeature, may prompt the operator to view the responses and make ajudgment on whether the response is due to an inconsequential feature ora primary feature that must be detected, or to simply determine that thehole is uninspectable or needs to be cleaned further before properinspection. The process of building the signature library can berepeated for additional features. Once the signature library is updatedthe next step depends upon the situation. For anomaly suppression thedata has already been acquired and the next step would be to proceed toStep 704. If desired, Step 702 could be repeated as well but it is notnecessary. For defect detection, the next step is Step 708.

Step 708 refers to the acquisition of additional signature responses andupdating the set of materials in the test set. This primarily applies tothe generation of the signature library for defect detection since theevaluation of the signature library through a performance study such asa probability of detection study should not incorporate measurementsthat were used for both creating the signature library and for assessingthe system performance. This may require obtaining additionalmeasurements on similar samples, possibly on electrical dischargemachined (EDM) notches instead of real cracks and possibly on similaralloy materials. For each feature and variation of interest, such asseveral scans where the axial position is changed so that the featureposition relevant to the sense element in an array is varied, thesignature response is added to the library. The filtered response (Step704) is repeated to verify that the desired feature response is enhancedby the updated library. Any of the material samples used to generatesignatures that were placed into the library of signature responses isconsidered part of a training set and is not part of the evaluation set.However, in cases where sufficient sample sets are not available, asubject matter expert may choose to keep the sample used to generate thesignature library within the set and retake the data to make it asindependent as practical. Consequently, this set of test materials, orthe evaluation, may be updated and the data acquisition step (Step 702)should be repeated.

This process allows the signature library to be generated or updated asneeded depending upon the inspection application. An initial signaturelibrary is not required for this process. Note that a signature librarymight be generated in advance, or during an inspection or both. Thedecision to modify a signature library must be made in the context ofhistorical, present and future performance. Note that if the data fromthe inspection method is digital and achievable, then a smallersignature library might be used during the performance of inspections toprovide an initial result to the operator, and later a larger signaturelibrary might be used to process the data again. Careful records of thesignature library contents used for each inspection and later analysisis needed to properly compare results and support decisions regardingfitness for service. Comparisons within a single part, for multipleparts or across fleets must ensure that the same signature libraries areused and the same inspection settings. This should be verified ifpractical on a standard. Having data on a standard, even if it is notused for calibration, is helpful to verify consistency acrossinspections.

It should be appreciated that while anomaly suppression was discussedabove in connection with corrosion detection, it may also be used toenhance detection of defects in bolt holes. For example, if a hole iscleaned and the cleaning produces a burr at an edge, the presence of theburr might produce a response that results in a false call or makes thehole uninspectable for conventional eddy current methods. One solutionis to extract signatures of typical burrs and other such features thatrequire identification and/or suppression, including debris, scratches,dinged edges, out of roundness, pits. Identification requires use of arange of signatures in a signature library that are used to process theinspection data to find a best match to the secondary feature. In onesuch embodiment, if the secondary feature is identified (e.g. a burr atan edge) then a confidence level might be reported regarding itsidentification and the likelihood that it is a burr and not a crack.This likelihood could be derived from the quality of the match to alibrary of burr responses as well as the match to a crack response toenable differentiation between burrs and cracks. In one such embodimentthe response from the secondary feature may be above a predeterminedthreshold that is established by locating EDM notches near known burrsin a training set of samples. In this embodiment, the threshold isdetermined above which the EDM notch response is statisticallydistinguishable from the burr (or other secondary anomaly) response. Ifthe secondary anomaly response is above the threshold, then an actionmight be prompted. Such actions might include, cleaning a bolt hole toremove the burr, extracting a new signature from a more representativesample to add to the suppression or detection library, or prompting theoperatore to view the responses of multiple channels to make arecommendation based on experience, or visual observations.

In the case of scratch detection and location within a bolt hole, theliftoff signatures from representative scratches may be used to identifya scratch and estimate depth of the scratch. If the scratch is shallow,the software may allow the inspection for cracks to proceed. If thescratch is deeper than a predetermined depth the inspection for cracksmay be halted or a measure of hole quality may require a maintenanceaction on the hole to improve the quality.

In one embodiment of this invention there are multiple primary (damageor quality) features that may require enhancement using a signaturelibrary and multiple secondary features (inconsequential features thatonly obscure the primary features). In one such embodiment, a subset ofprimary and secondary features are processed using associated signaturelibraries to enhance the primary responses and suppress the secondaryresponses. In one such embodiment, for features that are not processed(often called filtered) with a signature library, a multivariate inversemethod is used to estimate a property where the spatial or temporalresponse of the property is perturbed by the presence of the feature ofinterest and is apparent beyond a detection threshold without the needfor a signature library. In one such example, the feature is thelocation of an edge of a layer in a bolt hole for a secondary feature,or debris between layers or a burr resulting from cleaning of the hole.In another such example the primary feature is the thinning of a metallayer in a joint caused by corrosion that must be distinguished from afastener response.

An eddy current array sensor such as sensor 400 (FIG. 4A) may have afunctional symmetry such that essentially identical responses areobtained regardless of whether a defect is at a top edge or a bottomedge of a layer. Accordingly, a signature library may not requiredifferent signatures based on whether the inspection data is proximal toa top edge or a bottom edge. In some embodiments one row of sensingelements is located in the center of a single rectangular drive and thesensor is scanned in the circumferential direction and then incrementedin the axial direction to build an image of the entire internal surfaceof a bolt hole. The response to EDM notches (or other defects) of thesame size produces substantially the same signature shape when theleading drive conductor passes over the EDM notch for the top edge forthe nearest sensing element when compared to the response of the nearestsensing element for a bottom edge EDM notch when the trailing driveconductor crosses the EDM notch. The introduction of such symmetry maybe used to reduce the number of signatures needed in the signaturelibrary which in turn may reduce the needed processing time. Note thatthis same approach could apply to an edge, such as the edge of abulkhead that is relatively thin where the sensor is at 45 degrees andis scanned in a direction parallel to the top edge.

Section VI—Modular Test Specimen Set for Holes

The adequacy of a set of test specimens for an NDE equipment performancestudies can be a major limitation with regards to the application of theperformance study results. A reconfigurable set of inspection plates 905is described with reference to FIGS. 9A-9D. Inspection plates 905 may bepart of a larger set of inspection plates (not shown). The set ofinspection plates may be reconfigured numerous ways into various testspecimens by, for example, (a) changing the order of the plates in thetest specimen, (b) rotating a plate about an axis of test-hole symmetry,(c) flipping a plate over, (d) removing a plate, and (e) adding a plate.The test specimen may be used in a performance study such as aprobability of detection (POD) study to analyze and quantifyeffectiveness of an inspection technology for hole inspection. Such aperformance study provides a basis to quantify detection statistics inaddition to signal levels related to various features, instrument noiselevels in holes in good condition, and other metrics such as estimatingtime required for inspection.

FIG. 9A shows a test specimen 900 with rectangularly symmetricinspection plates 905. While rectangular plates are discussed primarilyin connection with FIGS. 9A-9D it should be appreciated that many of theaspects described apply similarly to circularly symmetric, squaresymmetric or any other rotationally symmetric set of inspection plates.

Each plate 905 includes multiple test holes 901 which are locatedsymmetrically about two-axes for rectangular plates. For circularlysymmetric plates test holes may be equally spaced through 360° forrotationally symmetric plates, allowing them to align concentricallywith holes on other plates when any plate is flipped and or rotated. Theplates may be of various thicknesses and materials.

Each test hole 901 on each plate may or may not have a feature ofinterest 910 such as a crack or electrical discharge machining (EDM)notch (see FIG. 9B). Manufactured features of interest 910 may include(i) features of types that would require actions such as hole repair orcomponent replacement in sizes both less than and greater than themaximum acceptable features size for the target application, and (ii)anomalies, or features of types that do not necessarily require actionbut have historically been mistaken for features that do require actionin past inspections. Examples of features that require action includebut are not limited to cracks and EDM notches used to mimic presence ofcracks. Examples of anomalies include but are not limited to burrs,dents, dings, gouges, metal-shim-contamination, out-of-roundness, pits,and scratches. Such features may be located at various positions of atest hole, such as at an edge, within the bore, and at variouscircumferential positions.

A specimen is formed by stacking two or more plates 905 and securing thestack together with an alignment tool so that corresponding holes oneach plate in the specimen are axially aligned. For rectangularlysymmetric plates, each plate may be installed in the test specimen stackin four orientations including nominal, rotated 180 degrees, flipped,and rotated 180 degrees and flipped to vary the locations of features ofinterest in various test specimens. A square-symmetric plate would alsopermit 90 degree rotation, while a circular symmetric plate would allowfor rotational increments of n/360 degrees, where n is the number ofcircularly symmetric holes at a given radius. (Note, a circularlysymmetric plate could also be designed with n groups of holes where eachgroup forms a straight line of holes.) A wide variety of specimens maybe formed by using different combinations of inspection plates andflipping and rotating the member plates.

Note that for plates with circularly symmetric hole locations holes maybe placed at multiple radii. Different hole diameters may be used, forexample, at each radii.

The alignment tool ensures that the respective test holes of the variousinspection plates are held coaxial to one another. In FIG. 9A thealignment tool is achieved by fastener and fastener holes 909. Suchfastener holes may also have the same symmetry as test holes 901. Insome embodiments holes different than the inspection holes are used forthe fasteners to clamp the stack of plates together. In someembodiments, certain test holes are arbitrarily selected for securingwith fasteners 909. In this way, the number of test holes combinationsmay be further increased. In some other embodiments the alignment toolcomprises a frame and a clamp. The frame secures the inspection platesin lateral alignment while the clamp prevents the plates from separatingfrom one another in the direction of the test hole axes. The frame mayalso serve as and edge blocker, preventing the sides of the plates frombecoming visible to the inspector. This may prevent the inspector fromrecognizing features that would not necessarily be known in an actualinspection such as the number of layers in the hole. Also such an edgeblocker may block from the inspector any markings on the plates used toidentify the plate or its orientation.

The perimeter of the test plates may be cut such that the perimeter issymmetric with each permitted rotation. This occurs naturally if asquare perimeter is used for plates with square-symmetric holes, orrectangular perimeter is used for plates with rectangular-symmetricholes. In the case of circularly-symmetric test holes, the perimeter ofsuch test plates may be cut such n/360 degree perimeter rotationalsymmetry may be achieved in any suitable way. For example, an n-sidedregular polygon shape may be used.

Plates may be stacked with a base fixture 903 with legs 904, andtop-plate 902 to (i) mask any features on the first plate of plates 905to the inspector and 2) provide free space behind the bottom plate forthe inspection tool to pass through the hole. Base fixtures 903 and topplate 902 have matching hole diameters to the inspection sample. Topplate 902 may be countersunk to include flat-head fasteners 909 to clampthe inspection sample to the base fixture while still providing anunobstructed inspection surface.

Markings 908 along the edge of the plates 905 may be used to identifyboth the inspection plate and its orientation so that the specimenconfiguration can be easily recorded and later reproduced. The basefixture 903 may provides a reference to record plate type 907, ID 908,and orientation 906. For example, for rectangular plates, the basefixture is marked on the left and center of the front edge and eachplate is marked on the left, right, and center of the front and backedges 908, and for rotationally symmetric plates, edge markings are atpredetermined angular positions.

Example markings are described for rectangular plates: The front centermarking on the base fixture 906 includes the four numerals ‘1’, ‘2’,‘3’, and ‘4’ equally spaced and centered within the front side. Wheninstalled on the base fixture in the nominal orientation, the platefront center marking will vertically align with the ‘1’ numeral of thebase fixture. When installed on the base fixture in the rotatedorientation, the plate back center marking (now rotated to the front)will vertically align with the ‘2’ numeral of the base fixture.Naturally, the plate front and back center markings will align with thenumerals ‘3’ and ‘4’ when flipped or rotated and flipped, respectively,providing a mechanism to identify plate orientation. The left frontmarking on the base fixture 907 includes unique one-characteralphanumeric identifiers equally spaced for each plate type (materialand thickness combination). When installed on the base fixture in thenominal orientation, the plate left front marking will vertically alignwith the associated one-character alphanumeric identifier for that platetype. The plate front right marking will be equally spaced from thecenter of the front face. Likewise, the left and right back markingswill share the horizontal position of the left and right front markingssuch that one of the four markings will vertically align with the basefixture marking for that plate type in the four orientations. The foursymmetric plate-type markings 908 uniquely identifies each sample plateby symbol of marking. The first instance of one plate type may use asingle dot marking. The next instance may use a two-dot marking.Additional symbols and combinations of symbols may be used. Symbols mustbe vertically symmetric and left-right symmetric or mirrored across thecenter of the marked face such that an operator could not determinewhich orientation a plate is in based on differences between the fourplate-type markings.

Each inspection plate may be used in multiple inspection sample stacks.The detection statistics will account for this condition whereapplicable. This method may be used to quantify differences in signalfrom the same size feature 910 when it is at the top of the hole, at thebottom of the hole, near an interface with a layer of the same material,near an interface with a layer of a different material, and each ofthese conditions when the feature layer thickness is small or largerelative to the feature size.

Plates may be used in conjunction with simple real crack specimens 911by including real crack specimens in the inspection sample stack. Inthis way, real cracks can be manufactured in simple, low-cost specimenswhich match the layer material and thickness of interest, independent ofthe plate design.

Inspection samples may be used with an additional fixture to orient themon their side, or inverted, to mimic inspection on actual components,for example, aircraft, where many bolt holes are on the sides orbottom-side of the aircraft.

It should be appreciated that the foregoing can be generalized to msymmetric axes. The rectangular case represents two symmetric axes (1rotation and 1 flip). The square case represents three symmetric axes (2rotations and 1 flip).

Section VII—Closing Discussion

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated that various alterations,modifications, and improvements will readily occur to those skilled inthe art.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andscope of the invention. Accordingly, the foregoing description anddrawings are by way of example only.

The above-described embodiments of the present invention can beimplemented in any of numerous ways. For example, the embodiments may beimplemented using hardware, software or a combination thereof. Whenimplemented in software, the software code can be executed on anysuitable processor or collection of processors, whether provided in asingle computer or distributed among multiple computers.

Further, it should be appreciated that a computer may be embodied in anyof a number of forms, such as a rack-mounted computer, a desktopcomputer, a laptop computer, or a tablet computer. Additionally, acomputer may be embedded in a device not generally regarded as acomputer but with suitable processing capabilities, including a PersonalDigital Assistant (PDA), a smartphone or any other suitable portable orfixed electronic device.

Also, a computer may have one or more input and output devices. Thesedevices can be used, among other things, to present a user interface.Examples of output devices that can be used to provide a user interfaceinclude printers or display screens for visual presentation of outputand speakers or other sound generating devices for audible presentationof output. Examples of input devices that can be used for a userinterface include keyboards, and pointing devices, such as mice, touchpads, and digitizing tablets. As another example, a computer may receiveinput information through speech recognition or in other audible format.

Such computers may be interconnected by one or more networks in anysuitable form, including as a local area network or a wide area network,such as an enterprise network or the Internet. Such networks may bebased on any suitable technology and may operate according to anysuitable protocol and may include wireless networks, wired networks orfiber optic networks.

Also, the various methods or processes outlined herein may be coded assoftware that is executable on one or more processors that employ anyone of a variety of operating systems or platforms. Additionally, suchsoftware may be written using any of a number of suitable programminglanguages and/or programming or scripting tools, and also may becompiled as executable machine language code or intermediate code thatis executed on a framework or virtual machine.

In this respect, the invention may be embodied as a computer readablemedium (or multiple computer readable media) (e.g., a computer memory,one or more floppy discs, compact discs, optical discs, magnetic tapes,flash memories, circuit configurations in Field Programmable Gate Arraysor other semiconductor devices, or other tangible computer storagemedium) encoded with one or more programs that, when executed on one ormore computers or other processors, perform methods that implement thevarious embodiments of the invention discussed above. The computerreadable medium or media can be transportable, such that the program orprograms stored thereon can be loaded onto one or more differentcomputers or other processors to implement various aspects of thepresent invention as discussed above.

In this respect, it should be appreciated that one implementation of theabove-described embodiments comprises at least one computer-readablemedium encoded with a computer program (e.g., a plurality ofinstructions), which, when executed on a processor, performs some or allof the above-discussed functions of these embodiments. As used herein,the term “computer-readable medium” encompasses only a computer-readablemedium that can be considered to be a machine or a manufacture (i.e.,article of manufacture). A computer-readable medium may be, for example,a tangible medium on which computer-readable information may be encodedor stored, a storage medium on which computer-readable information maybe encoded or stored, and/or a non-transitory medium on whichcomputer-readable information may be encoded or stored. Othernon-exhaustive examples of computer-readable media include a computermemory (e.g., a ROM, a RAM, a flash memory, or other type of computermemory), a magnetic disc or tape, an optical disc, and/or other types ofcomputer-readable media that can be considered to be a machine or amanufacture.

The terms “program” or “software” are used herein in a generic sense torefer to any type of computer code or set of computer-executableinstructions that can be employed to program a computer or otherprocessor to implement various aspects of the present invention asdiscussed above. Additionally, it should be appreciated that accordingto one aspect of this embodiment, one or more computer programs thatwhen executed perform methods of the present invention need not resideon a single computer or processor, but may be distributed in a modularfashion amongst a number of different computers or processors toimplement various aspects of the present invention.

Computer-executable instructions may be in many forms, such as programmodules, executed by one or more computers or other devices. Generally,program modules include routines, programs, objects, components, datastructures, etc. that perform particular tasks or implement particularabstract data types. Typically the functionality of the program modulesmay be combined or distributed as desired in various embodiments.

Also, data structures may be stored in computer-readable media in anysuitable form. For simplicity of illustration, data structures may beshown to have fields that are related through location in the datastructure. Such relationships may likewise be achieved by assigningstorage for the fields with locations in a computer-readable medium thatconveys relationship between the fields. However, any suitable mechanismmay be used to establish a relationship between information in fields ofa data structure, including through the use of pointers, tags or othermechanisms that establish relationship between data elements.

Various aspects of the present invention may be used alone, incombination, or in a variety of arrangements not specifically discussedin the embodiments described in the foregoing and is therefore notlimited in its application to the details and arrangement of componentsset forth in the foregoing description or illustrated in the drawings.For example, aspects described in one embodiment may be combined in anymanner with aspects described in other embodiments.

Also, the invention may be embodied as a method, of which an example hasbeen provided. The acts performed as part of the method may be orderedin any suitable way. Accordingly, embodiments may be constructed inwhich acts are performed in an order different than illustrated, whichmay include performing some acts simultaneously, even though shown assequential acts in illustrative embodiments.

For the purposes of describing and defining the present disclosure, itis noted that terms of degree (e.g., “substantially,” “slightly,”“about,” “comparable,” etc.) may be utilized herein to represent theinherent degree of uncertainty that may be attributed to anyquantitative comparison, value, measurement, or other representation.Such terms of degree may also be utilized herein to represent the degreeby which a quantitative representation may vary from a stated reference(e.g., about 10% or less) without resulting in a change in the basicfunction of the subject matter at issue. Unless otherwise stated herein,any numerical values appeared in this specification are deemed modifiedby a term of degree thereby reflecting their intrinsic uncertainty. The“substantially simultaneous response” means responses measured within 1second of one another.

Use of ordinal terms such as “first,” “second,” “third,” etc., in theclaims to modify a claim element does not by itself connote anypriority, precedence, or order of one claim element over another or thetemporal order in which acts of a method are performed, but are usedmerely as labels to distinguish one claim element having a certain namefrom another element having a same name (but for use of the ordinalterm) to distinguish the claim elements.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having,” “containing,” “involving,” andvariations thereof herein, is meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

What is claimed is:
 1. An apparatus comprising: a test specimen having aplurality of inspection plates, each plate having a plurality of testholes symmetric about a first axis of said plate; and an alignment toolsecuring the plurality of inspection plates such that corresponding testholes on each plate are axially aligned, wherein a first plate among theplurality of inspection plates has a defect feature at a first test holeamong the plurality of test holes for said first plate.
 2. The apparatusof claim 1, wherein the plurality of test holes on each of the pluralityof inspection plates comprises a first plurality of test holes at afirst radius from the first axis and a second plurality of test holes ata second radius from the first axis, the second radius greater than thefirst radius.
 3. The apparatus of claim 2, wherein the first pluralityof test holes have a first hole diameter and the second plurality oftest holes have a second hole diameter different that the first holediameter.
 4. The apparatus of claim 1, wherein the plurality of testholes on each of the plurality of inspection plates are each at a firstradius from the first axis and are equal angularly spaced about thefirst axis.
 5. The apparatus of claim 4, wherein each of the pluralityof inspection plates has a first face, a second face and an exterioredge, the exterior edge having an indicator identifying which face isthe first face.
 6. The apparatus of claim 1, wherein the plurality oftest holes on each plate of the plurality of inspection plates aresymmetric about a second axis of said plate, the first and second axesbeing perpendicular to one another.
 7. The apparatus of claim 1, whereinthe plurality of plates have a same thickness.
 8. The apparatus of claim1, wherein the plurality of inspection plates of the test specimen are asub-plurality of a test set of inspection plates and all plates in thetest set have test holes.
 9. The apparatus of claim 1, wherein thedefect feature is of a type selected from a group consisting of a crackand an electro discharge machining (EDM) notch.
 10. The apparatus ofclaim 9, wherein the defect feature is at a corner location of the firsttest hole.
 11. The apparatus of claim 9, wherein the defect feature ismidbore in the first test hole.
 12. The apparatus of claim 1, whereineach of the plurality of inspection plates has unique identifyingmarkings along its edge, and the test specimen further comprises an edgeblocker that surrounds the perimeter of the plurality of inspectionplates such that the plates' edges are not visible.
 13. An apparatuscomprising: a test specimen having a plurality of inspection plates,each plate having a plurality of test holes symmetric about a first axisof said plate and symmetric about a second axis of said plate, the firstand second axes being perpendicular to one another; and an alignmenttool securing the plurality of inspection plates such that correspondingtest holes on each plate are axially aligned, wherein a first plateamong the plurality of inspection plates has a defect feature at a firsttest hole among the plurality of test holes for said first plate. 14.The apparatus of claim 13, wherein the plurality of inspection platesfurther include a plurality of alignment holes symmetric about the firstaxis of said plate and symmetric about the second axis of said plate;and the alignment tool includes a plurality of fasteners, each fastenerpassing through a respective alignment hole of each of the plurality ofinspection plates.
 15. The apparatus of claim 14, wherein the alignmenttool further includes a base to which the plurality of inspection platesare secured by the plurality of fasteners.
 16. The apparatus of claim15, wherein each of the plurality of inspection plates is approximatelyrectangular in shape, has on a first edge a first marking offset fromthe first axis by a first amount, and has on a second edge, opposite thefirst edge, a second marking offset from the first axis by a secondamount less than the first; and the base has four orientation indicatorspositioned on the base such that for each plate either the respectivefirst marking or the respective second marking aligns with one of thefour orientation indicators depending on the orientation of said plate.17. The apparatus of claim 13, wherein the alignment tool includes aplurality of fasteners, each fastener passing through a respective testhole of each of the plurality of inspection plates, the plurality offasteners being of a smaller number than a number of axially alignedtest holes in the specimen.
 18. A method of determining performance of ahole inspection system, the method comprising acts of: (i) providing atest set of inspection plates, each plate in the test set having aplurality of test holes symmetric about a first axis of said plate, afirst plate in the test set having a defect feature at a first test holeamong its plurality of test holes; (ii) securing with an alignment toola first subset of plates in the test set into a first test specimen suchthat corresponding test holes on each plate in the first subset areaxially aligned, the first subset of plates including the first plate;(iii) performing an inspection procedure with the hole inspection systemin each of the plurality of test holes in the first test specimen; (iv)storing inspection results for each of the plurality of test holes inthe first test specimen; and (v) determining performance of the holeinspection system based at least in part on whether the results detectedthe defect feature in the first test hole of the first plate.
 19. Themethod of claim 18, further comprising acts of: (vi) securing with thealignment tool a second subset of plates in the test set into a secondtest specimen, wherein the second subset of plates is different from thefirst subset in at least one of (a) an order of plates, (b) a platecommon to both the first and second subsets of plates is rotated aboutits first axis in the second test specimen relative to the first testspecimen, (c) a plate common to both the first and second subsets ofplates is flipped over in the second test specimen relative to the firsttest specimen, (d) a plate in the first subset is absent from the secondsubset, and (e) a plate in the second subset is absent from the firstsubset; and (vii) repeating acts (iii) and (iv) for the second testspecimen, wherein act (v) is performed based on inspection results fromboth the first and second test specimens.
 20. The method of claim 18,further comprising acts of: (vi) securing with the alignment tool asecond subset of plates in the test set into a second test specimen,wherein the second subset of plates is different from the first subsetin at least a plate common to both the first and second subsets ofplates is flipped over in the second test specimen relative to the firsttest specimen; and (vii) repeating acts (iii) and (iv) for the secondtest specimen, wherein act (v) is performed based on inspection resultsfrom both the first and second test specimens.